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The starting date:
One begins to count Sefiras Haomoer on the second night of Pesach, which is the 16th of Nissan. [This applies in all areas, even the Diaspora.] This is because the Omer offering was brought on the 16th of Nissan, and one begins to count from the day that the Omer offering is harvested.
At night versus day:
The Omer must be counted specifically during the night of the 16th, and so too during every subsequent night of Sefira. It is invalid to count the Omer by day. However, some Poskim rule that if one forgot, or purposely did not count, at night then he is obligated to count by day. [See Halacha 17 for the full details of this matter and the final ruling!]
When at night?
The entire night, up until Alos/daybreak, is valid for counting the Omer. However, initially, it is a Mitzvah Min Hamuvchar to Daven Maariv immediately after nightfall and then count the Sefira immediately afterwards. [This applies towards all the nights of Sefirah. One is to always try to count the Sefira within the first half hour of the night.] If one forgot and did not count the Sefira at the beginning of the night then if he remembered to do so prior to Alos Hashachar, he is obligated to count [with a blessing]. [Initially one is to count the Omer prior to midnight, if he was unable to do so at the beginning of the night. Furthermore, the earlier one counts, the better, as delaying the count causes the side of evil to increase its nurture from Holiness.]
May one count before Maariv? It is permitted to count the Sefira before Maariv [after nightfall]. This applies even on Motzei Shabbos, when one extends the Shabbos into the night [and does not Daven Maariv until later]. Nevertheless, it is proper to precede the Davening of Maariv to the counting of the Omer.
May one count the next day’s count after Plag Hamincha? One may not begin to count Sefira after Plag Hamincha, and rather must wait until the beginning of night. Thus, even those who Daven Maariv after Plag Hamincha are not to count the Omer after Maariv in Shul and rather every person is to count in his home, when the time arrives. This applies even on Erev Shabbos. If one forgot and said a blessing and counted after Plag Hamincha, prior to night, then he must repeat the count at nightfall, although without a blessing. [If one did not do so, see Halacha 22 regarding if one may continue counting with a blessing on the following nights.] If one counted the next day’s count prior to Plag Hamincha, then one must repeat the counting at night with a blessing. [Those communities who are accustomed to count the sefirah after Pelag Hamincha, and one is unable to change the custom, one is to accustom them to count Sefirah also the next day after Davening.]
May one count by Bein Hashmashos? It is disputed amongst Poskim as to whether one may initially begin counting from after sunset or if he must wait until after nightfall. Practically, it is proper for every person to avoid counting the Omer during Bein Hashmashos, until after nightfall. Nevertheless, in the event that one accidently counted the Sefira by Bein Hashmashos he has fulfilled his obligation and is not required at all from the letter of the law to repeat the count after nightfall, even without a blessing. (Nevertheless, it is proper to recount after nightfall without a blessing even in such a case.) [If one did not do so, see Halacha 22 that he may nevertheless continue counting with a blessing the following nights.]
May one recount with a blessing if he heard someone else count by Bein Hashmashos or Plag Hamincha? See Halacha 15!
When to count in the Diaspora on the second night of Pesach: On the second night of Pesach one begins to count Sefiras Haomer immediately after Maariv. However, some Poskim say that those who follow mysticism should count after finishing the entire Seder on the second night in the Diaspora. [Other Poskim however negate this and rule one must count after Maariv, before the Seder, both according to Nigleh and Nistar.] Practically, one who proceeds to say the blessing and count immediately after Davening, is preceded with Divine blessing.
The Sefira is counted at night immediately following Maariv. It is a Mitzvah Min Hamuvchar to initially Daven Maariv immediately after nightfall and then count the Sefira immediately afterwards. One is to avoid counting prior to nightfall, and certainly may not count by Plag Hamincha.
Are those who follow Rabbeinu Tam to be stringent not to count until after Tzeis Hakochavim of Rabbeinu Tam?
Those who are stringent to follow the time of Rabbeinu Tam regarding all nighttime dependent matters, then if they follow Rabbeinu Tam even regarding Rabbinical matters, they are to wait until after Tzeis Hakochavim of Rabbeinu Tam to recite Sefiras Haomer. If, however they only follow Rabbeinu Tam regarding Biblical matters, they may recite Sefiras Haomer after Tzeis Hakochavim of the Geonim, after Davening Maariv. Some however are stringent in all cases.
When in Maariv is one to count?
Various customs exist as to the positioning of Sefiras Haomer after the Maariv prayer. Some communities are accustomed to reciting it after Kaddish Tiskabel, before Aleinu. Other communities however recite it after Aleinu. Practically, the Chabad custom, based on Admur in the Siddur, is to recite it after Kaddish Tiskabel, prior to Aleinu.
Kiddush Levana: The custom is to precede Sefiras Haomer to Kiddush Levana.
What is one to do if he is by a Minyan that counts after Aleinu?
If one is Davening by a Minyan who counts the Sefirah after Aleinu, he is to delay his count and count together with the Minyan after Aleinu. If one is an Avel, or has a Yartzite, and has to say Kaddish, he is to say only one Kaddish after both Aleinu and the Sefira. If, however the congregation does not agree to this, then he may say the Kaddish after Aleinu, before Sefirah, and also after the Sefirah, if so is their custom.
If one will not be able to Daven Maariv with a Minyan immediately after nightfall, what is he to do?
If one will not be able to Daaven Maariv with a Minyan immediately after nightfall, he is to Daaven Maariv with a Minyan later on and is to delay the Sefira until after Maariv. Accordingly, if one has a set Minyan he attends later on into the night, he is to delay the Sefirah and say it together with the Minyan. If however one anyway plans to Daven Maariv without a Minyan later on at night, some Poskim rule he is to count the Sefirah immediately after nightfall, and not wait until he Davens Maariv. Other Poskim however rule he is to delay the Sefirah even in such a case, and so is implied from Admur.
If one attends a set Minyan that Davens Maariv later at night, should he change his schedule during the period of Sefirah to Daven with a Minyan by Tzeis?
It is proper during the duration of Sefirah to join a Minyan that Daavens Maariv immediately after nightfall, and thus benefit from saying the Sefirah immediately after nightfall with a Minyan, as well as from saying Kerias Shema immediately at night.
Is it proper to count the Omer together with a Minyan?
It is proper to count the Sefira together with the Minyan. [Thus, one should not leave the Minyan for Maariv until after Sefira.]
If the Minyan is holding by Sefira and one has not yet begun Maariv is he to recite Sefira with the Tzibur?
If one came late to Shul and the congregation has reached Sefiras Haomer prior to him beginning Maariv, then in the event that he will not be Davening with another Minyan, some Poskim rule that he is to count Sefira together with the Minyan and Daven Maariv afterwards,. The same applies if he skipped Kerias Shema in order to begin Shemoneh Esrei together with the Minyan, that he should count Sefira together with the Minyan, prior to reciting Shema. Other Poskim however rule that one is to always precede Maariv to counting Sefira, even if it means that he will lose the opportunity to count with a Minyan. Practically, if one suspects he may forget to count Sefira after his private Davening, he is to count with the Minyan before Maariv.
If one only remembered to count that day’s Sefira during Bein Hashamshos is he to still count?
Ø Example: On the 32nd day of Sefira one remembered that he did not yet count and it is already after sunset, is he to count the Sefira?
Yes. If it is still prior to nightfall then one is to count the Sefira without a blessing. It is disputed as to whether one may continue to count with a blessing on the coming nights. See Halacha 22!
Counting by day after Shacharis:
It is customary in certain communities for the Chazzan of Shacharis to repeat the Sefirah after Davening, and have the congregation say it after him, in order to save one who forgot to count the previous night.
Counting several times:
Some are accustomed to count the Omer several times at night and during the day in order so the count remains constantly on their minds.
Writing the Sefira in a letter/document:
Some are accustomed to mention the day of Sefira when writing a letter during the Omer period. [This custom was not followed by the Chabad Rabbeim.]
One who is attending a Minyan that counts by Bein Hashmashos:
If one who is particular to only say the Sefira after nightfall [as explained in Halacha C] is Davening Maariv with a Minyan that Davens by Beis Hashmashos and also counts the Sefira by Bein Hashmashos, then it is proper for him to count the Sefira together with them without a blessing, on the basis of a Tnaiy [stipulation], and to then count later on at night with a blessing. The Tnaiy that he should make at the time of the count is as follows: “If I forget to count at night, I am relying on this count to fulfill my obligation. If however I will not forget, then I am now having in mind to not fulfill my obligation with this count.” It suffices to think this stipulation in one’s mind, and it is not necessary to be verbalized. If however, one did not explicitly have in mind to make this Tnaiy, to not fulfill his obligation with this count if he remembers at night, and rather he casually counted without any specific intent, then although he did not say a blessing at this time and did not answer Amen after the Chazzan’s blessing, nevertheless, he may not count at night with a blessing, as perhaps he has already fulfilled his obligation. [He is however to recount without a blessing.]
 Admur 489/3
 Siddur Admur and all Poskim brought next; The Poskim however discuss the question as to why we count the Omer with a blessing in the Diaspora on the night of the 16th, if it is Sfeika Deyoma, and we perform the Seder that night. See: M”A 489/1; Chok Yaakov 489/2; Tamim Deim of Raavad; Mor Uketzia 489; Sheilas Yaavetz 2/83; Nimueki Orach Chaim 489; Zichron Yehuda 147; Piskeiy Teshuvos 489/1
 The source that the Omer was brought on the 16th: As the verse states “On the day after Shabbos you shall wave [the Omer]” and the Sages received a tradition that this refers to the day after the first day of Pesach. [Admur ibid]
The source to begin the count on the day of the offering of the Omer: The verse states “Mehachel Charmesh Kama Tachel Lispor”, which means that from the time of the harvest one is to begin the count of the Omer. Now, the beginning of the harvest is prior to when the Omer is offered and not afterwards. Hence, from here we learn that from this day itself [the 16th] one begins the counting and not the day afterwards. [Admur ibid]
 Admur 489/3
 The source: (Although the Omer offering had to be brought during the actual day), nevertheless, the counting of the Omer could not begin during the day, as the verse states “Temimos Tehiyehna”, which means complete, and if one were to begin the count during the day of the 16th then that day would be incomplete. Hence, in order to satisfy the requirement of complete days the count must begin at night of the 16th, prior to daybreak. Now, although this concept only applies to when one is to start the counting on the first day, nevertheless, we apply it to also all the other days, that the counting is only valid at night, as it can be assumed that the counting of all the days are similar to each other in law. [Admur ibid]
 Stam opinion in Admur ibid; Rabbeinu Tam in Megillah ibid
 2nd opinion in Admur ibid; Bahag Atzeres 30/4; Meiri Megillah 2 It is evident from Admur ibid and other Poskim [Nehar Shalom in Biur Halacha 489/8 “Belo Bracha”] that even according to this opinion, there is an obligation to count Sefira specifically at night, and it is only if one forgot that he can make it up during the day. See Biur Halacha 489/8 “Belo Bracha” that the Nehar Shalom holds that according to this opinion the Mitzvah applies for a full 24 hours, even though the main Mitzvah is at night, although the Biur Halacha himself concludes that perhaps even according to this opinion, the day is only a Tashlumin of the night.
 Admur 489/3; Siddur Admur; Michaber 489/1
 It is a Mitzvah Min Hamuvchar, even during the year, when Sefira is not recited, to Daven Maariv immediately following nightfall in order to say Kerias Shema immediately after nightfall. [Michaber 235/3-states Lechatchila one is to do so; Peri Megadim A”A 9 states it is merely a Mitzvah Min Hamuvchar]. The novelty here in this law is that it is likewise a Mitzvah Min Hamuvchar regarding the Mitzvah of Sefira to count immediately after nightfall, and hence one is to count the Sefira immediately after Maariv, which itself is prayed immediately after nightfall.
 The reason: As one is to count the Sefira as close as possible to the beginning of the night. [Admur ibid] The reason for this is in order so the count be Temimos and cover the entire 24 hour period of that day. [M”B 489/2]
 Admur ibid does not make any differentiation; Siddur Yaavetz Chayeh Adam 131/2; See Piskeiy Teshuvos 489/2
The reason: As every night is a separate Mitzvah, and the concept of Temimos applies to each night’s Mitzvah individually. [Piskeiy Teshuvos ibid]
Other opinions: Some Poskim rule it is only necessary to count in the beginning of the night on the first night of Sefirah, as the concept of Temimos only applies on the first night. [Ran end of Pesachim, brought in beis Yosef 489]
 Moreh Baetzba 7/216
 See Halacha 19 in Q&A regarding the calculation of Alos.
 Tzemach Tzedek glosses on Siddur 36; Sichas Acharon Shel Pesach 5717; See Vayaged Moshe p. 281-283; Glosses of Rav Raskin p. 624
 Kaf Hachaim 489/12
 Admur 489/16
 Admur ibid; M”A 489/7; Siddur Beis Oved 3; Kaf Hachaim 489/43
 Admur ibid; M”A 489/7
The reason: As nevertheless it is already night and is not considered the previous day but rather the next day. [Admur ibid]
 Admur ibid; Chok Yaakov 489/16; Biur Halacha 489/1 “Achar” rule it must be preceded due to Tadir
The reason: As Maariv is Tadir [i.e. common] while the counting of the Omer is only on occasion, and the rule follows that a more common Mitzvah receives precedence over a less common Mitzvah. [Admur ibid; Chok Yaakov ibid; Biur Halacha ibid; However from Admur ibid it is clear that we don’t fully rule this way, and that is why it is only “proper” to precede Maariv, however from the other Poskim it is evident that it is required from the letter of the law] Alternatively, the reason is because the Sefira must be said after nightfall, and in previous times it was common to Daven Maariv early. [Mur Uketzia 489, brought in Biur Halacha ibid; See Nimukei Orach Chaim 489; Igros Moshe 4/99; Piskeiy Teshuvos 489/2 footnote 12]
 Admur 489/12; Taz 489/6; Biur Hagr”a 489; M”B 489/16 in name of Poskim; These Poskim interpret Michaber 489/3 who writes that a congregation can count before night, that this refers to Bein Hashmashos, and not to Plag Hamincha
Other opinions: Some Poskim rule one may count Sefirah during Plag Hamincha, and that so is the intent of the Michaber ibid. [Chok Yaakov 489/13; Elya Raba 489/11; Chok Yosef 489/16; Mamar Mordechai 489/5; Nehar Shalom 489/3, brought in Biur Halacha 489/3 “Mibiod Yom” and Kaf Hachaim 489/48]
 As is accustomed in many areas during summer Shabbosim to Daven Maariv early, prior to Bein Hashmashos. [ibid]
 The reason: The reason for why one may not count from Plag Hamincha is because the main opinion follows those who rule that even according to Rav Yehuda Plag Hamincha is only considered night regarding the prayer obligations and not regarding other matters that are to be done at night. [Admur ibid]
 Admur ibid; Maharshal 13; Bach 489; Chok Yaakov 489/12
Other opinions regarding Erev Shabbos: Some communities are accustomed on Erev Shabbos to Daven Maariv early and recite Sefiras Haomer before nightfall. [See Maharshal 13; Leket Yosher in name of Terumos Hadeshen; Mateh Moshe 673 in name of Raavan; Aruch Hashulchan 489/7; Shraga Hameir 6/41; See Piskeiy Teshuvos 489/12 footnote 61]
 So writes Admur and Poskim ibid that the case is that a blessing was said together with the counting. However seemingly the same would apply even if a blessing was not said, so long as he had in mind to fulfill the Mitzvah. See coming footnotes and Biur Halacha there.
 The reason: The reason why one must repeat the count after nightfall if he accidently counted after Maariv of Plag Hamincha, even though he follows the opinion of Rebbe Yehuda who considers it night starting from Plag Hamincha, is because the main opinion follows those who rule [Rosh Brachos 1/1; Michaber 235/1] that even according to Rav Yehuda Plag Hamincha is only considered night regarding the prayer obligations and not regarding other matters that are to be done at night. [Admur ibid]
 Admur ibid; Olas Shabbos 489/2; Chok Yaakov 489/12; Kaf Hachaim 489/45
The reason: The reason for why a blessing is not recited a second time by nightfall if one accidently counted after Plag Hamincha is because the custom of the world [of those times] is to read the night Shema of Maariv from Plag Hamincha and onwards. This is because they follow those opinions [Rabbeinu Tam and Riy in Tosafus Brachos 2a] who hold that just like from Plag Hamincha begins the night regarding Davening Shemoneh Esrei of Maariv according to Rebbe Yehudah [in Brachos 26b] so too from Plag Hamincha and onwards it is considered like night regarding the reading of the Shema of Maariv. Following this approach, the same would apply to Sefiras Haomer, that according to the opinion of Rebbe Yehuda, starting from Plag Hamincha is considered like night regarding the Mitzvah of counting. [Admur ibid] Thus, in order to suspect for this opinion one does not repeat the blessing, as in their opinion one has already fulfilled the Mitzvah, and Safek Brachos Lihakel. [Vetzaruch Iyun regarding today that we follow the Sages, that night begins at nightfall, if one would have to repeat the counting with a blessing. See Biur Halacha ibid who rules that he may recount with a blessing being that Plag Hamincha is not night at all according to the final ruling.]
Other opinions: Some Poskim rule that one who counted during Plag Hamincha may repeat the blessing and counting at night, and the blessing is not in vain, as his previous count was not valid at all. [Biur Halacha 489/3 “Mibiod Yom”]
 Admur ibid
 Igros Kodesh 19/272 [printed in Shulchan Menachem 3/1]
 Admur 489/12-13; See Igros Kodesh 19/272 [printed in Shulchan Menachem 3/1]
 Some Poskim rule [Tosafus Menachos 66a; Smag 200 in name of Rashba; Rosh 10/40; Tur; See Michaber 489/3] that it is permitted to count the Omer with a blessing during Bein Hashmashos even though Bein Hashmashos is considered in question as to whether it is the beginning of the coming night or the end of the past day. Now, although it seems problematic to say “Today is the 2nd day” when in truth it may still be the 1st day, nevertheless, since today the counting of the Omer is only a Rabbinical command, therefore one may be lenient, as is always the rule regarding a doubt by a Rabbinical command. [Safek Derabanan Lihakel] Other Poskim [Tosafus ibid; Ran Pesachim 28; Taz 489/5] however argue on this and rule that Lechatchilah one may not enter himself into a doubt even by a Rabbinical command, and thus one is to delay the counting until after nightfall when it is definitely night. [Admur 489/12]
 So is done by the meticulous and so is proper to be followed by every Jew. [Admur ibid; Michaber 489/2] However the widespread custom [in those times] is like the first opinion to count the Omer during Bein Hashmashos, after sunset. [Admur ibid; Bach 489; Chok Yaakov 489/12]
What is one to do if he is Davening with a Minyan that is counting Sefira before Bein Hashmashos: See Halacha D!
Erev Shabbos: Some communities are accustomed on Erev Shabbos to Daven Maariv early and recite Sefiras Haomer before nightfall. [Maharshal 13; Leket Yosher in name of Terumos Hadeshen; Mateh Moshe 673 in name of Raavan; Aruch Hashulchan 489/7; Shraga Hameir 6/41; See Piskeiy Teshuvos 489/12 footnote 61]
 Admur 489/13; M”A 489/6
The reason: As he has already fulfilled his obligation, as although Bein Hashmashos is held in question if it is considered night, nevertheless by a Rabbinical command we are lenient in case of doubt. [Admur ibid; Elya Raba 489/10]
 Lit. Tov
 Admur ibid; [parentheses in original]; Elya Raba ibid; P”M 489 M”Z 5; Kaf Hachaim 489/41
The reason: It is proper to recount after nightfall without a blessing in order to suspect for those opinions who say that even today the counting of the Omer is a Biblical obligation [and in their opinion one does not fulfill his obligation in case of doubt]. [Admur ibid]
 Siddur Admur; See Shaar Hakolel 49/2; Piskeiy Teshuvos 489/1
 Admur ibid; Michaber 489/1
 Rameh Mipuno; Ramaz; Ramak; Chesed Leavraham brought by Chida in Chaim Sheol 2/10 and Machazik Bracha 489/2 [In Chaim Sheol ibid he concludes like this opinion, however in Birkeiy Yosef 489/5 he seems to negate it]; Birkeiy Yosef 489/5 that so is custom of Midakdikin; Yesod Veshoresh Havoda 9/8; Mishmeres Shalom Kudinov 36/1; Divrei Chaim; Maharshag 30 regarding if there will be a Minyan after the Seder; See Vayaged Moshe ibid that so was custom of Baal Shem Tov and his students, however this matter is under debate [See Piskeiy Teshuvos ibid footnote 3 and 6]; See Shaar Hakolel 49/2 in name of Tzemach Tzedek that the opinion brought here who holds to count after the Seder was not written by Admur but rather by his brother the Maharil, and when Admur saw this after it was printed he was very upset.
 The reason: As once one counts the Omer he is pronouncing that it is no longer Pesach, and this contradicts his later performance of the Seder. [See Chida ibid; Shaar Hakolel ibid]
 Rashash in Nehar Shalom p. 25, “One is not to delay the Sefirah even according to Kabbalah and on the contrary, one who does so causes Yenikas Hachitzonim.” [See however Piskeiy Teshuvos ibid footnote 6 in name of Divrei Chaim that the Rashash retracted this ruling later on, and how it does not apply to us]; Machazik Bracha 489/2 in name of Rashash [although concludes unlike him]; Birkeiy Yosef ibid seemingly concludes like Rashash; Minchas Elazar 4/18; Nimukei Orach Chaim 489; Shaar Yissachar 2; Aruch Hashulchan 489/11; Kaf Hachaim 489/6; See Shaar Hakolel ibid; Piskeiy Teshuvos ibid footnote 6
 The reason: As it is forbidden to eat prior to Sefira, and he also may come to forget to count afterwards. [Birkeiy Yosef ibid; Tzemach Tzedek in his glosses on Siddur; Poskim ibid]
 Minchas Yitzchak 6/45; 9/55; 10/42; See Birkeiy Yosef 489/12 and Kaf Hachaim 489/44 in name of Batei Kehuna 2/4 that there is no need to be stringent like Rabbeinu Tam regarding Sefira; Piskeiy Teshuvos 489/12
 See Kaf Hachaim ibid that so is custom in Tiberius; Mishneh Halachos 6/103
 Shaar Hakolel 49/7 that so was the directive of Admur; Chok Yaakov 489/20 [immediately after Kaddish Tiskabel]; Elya Raba 489/26; Chok Yosef 489/28; P”M 489 A”A 15 that he would say it before Vayiten Lecha; M”B 489/2 “Before Aleinu”; Darkei Chaim Veshalom 625 [before Aleinu]; Minhagei Berlin p. 144 [before Aleinu]; Likkutei Halevi [Minhagei Wotzberg] p. 12 [immediately after Shemoneh Esrei]; Minhagei Piruda 64 [before Aleinu]; Nivei Shalom Dinei Haomer 3 [before Aleinu]; Minhagei Na Amon [Alexandria] in name of Seder Hayom that one is to count before Aleinu; Nehar Mitzrayim Hilchos Omer 25 [After Kaddish Tiskabel]; Kav Chaim 48; Rivivos Efraim 1/334
 The reason: a) The Sefira is counted before Aleinu, versus afterwards, in order to be able to include Sefiras Haomer in the Kaddish that is said directly after Aleinu, as the Sefira requires a Kaddish afterwards [due to all the verses we say as part of Sefira] and it is forbidden to increase in Kaddeishim. [Shaar Hakolel 49/7; See Chikrei Haminhagim p. 158] b) In order to precede the Sefira as much as possible, and fulfill the Mitzvah of Temimos. [M”B ibid] c) Alternatively, it is said in Davening in order so the people who leave right after Aleinu do not forget to recite it. [Nehar Mitzrayim ibid] d) As Aleinu is always the closing prayer of the prayers, and we always insert all other matters beforehand, such as reading Megillah, Kinos etc. [Grac”h Na”ah, in letter printed in Kovetz Ohalei Sheim 7/43]
 Kaf Hachaim 489/102; Minhag Hagr”a [brought in Shaareiy Rachamim 489/2; Maaseh Rav Hashaleim Pesukei Dezimra 69Minhag Yerushalyim [brought in nehar Mitzrayim ibid and letter of Grac”h Na”h, printed in Kovetz Ohalei Sheim 7/43]; Minhag of majority of communities [recorded in Minchas Haomer 489/33 in name of Kovetz Hameasef 18/2]
 Shaar Hakolel 49/7 that so was the directive of Admur
 See Devar Yehoshua 2/13; Lehoros Nasan 1/28; Rivivos Efraim 2/127; Piskeiy Teshuvos 489/26; These Poskim discuss whether Kiddush Levana or Sefira should precede due to the contrast of the Tadir of Sefira versus Tadir of Kiddush Levana, and the fact that Sefira is possibly Biblical, although the custom is as stated above..
 Pashut as one is meant to say it with a Minyan, and the entire reason we say it before Aleinu is only because of the Kaddish; See also Igros Kodesh in next footnote
 Igros Kodesh 5/313 [printed in Shulchan Menachem 3/2] in a reply to Rav Yitzchak Dubov. This is despite the concerns brought earlier in Shaar Hakolel ibid regarding the prohibition to increase in Kaddeishim.
 Minchas Yitzchak 9/56; Igros Moshe 4/99; Shevet Halevi 6/53; Az Nidbaru 6/52; Mishneh Halachos 8/193; Piskeiy Teshuvos 489/2
 The reason: a) As Maariv is Tadir and receives precedence. [Admur ibid; Chok Yaakov ibid; Biur Halacha ibid] b) The Mitzvah is to be done with a Minyan. [Shelah Miseches Pesachim Ner Mitzvah, brought in Chok Yaakov 489/16] See also Shevet Halevi ibid
 Shevet Halevi ibid
 See Piskeiy Teshuvos ibid footnote 14
 Shelah Hakadosh p. 143b; Chok Yaakov 489/16; Shalmei Tzibbur p. 296; Kaf Hachaim 489/42
 Shelah brought in Chok Yaakov 489/16 “Sefira with a Minyan”; Beir Heiytiv 489/20; Siddur Yaavetz “Sefira with Rov Am”; Minchas Yitzchak 9/56; See Piskeiy Teshuvos 489/3
 See Piskeiy Teshuvos 489/3
 Meiam Loez Vayikra p. 278; Moed Lekol Chaiy; Divrei Hageonim 39 in name of Derech Hachaim; Minchas Yitzchak 9/56; Orchos Rabbeinu 2/94 that so ruled Chazon Ish
 The reason: As it is proper to count the Omer with a Minyan [Poskim ibid] and one should not separate from the Tzibur. [ibid]
 Minchas Yitzchak 9/56; Orchos Rabbeinu 2/94 that so ruled Chazon Ish; To note however that many are accustomed never to skip the order of Maariv and so is the Chabad custom. [Kaf Hachaim 236/22; Ketzos Hashulchan 27 footnote 27]
 Teshuvos Vehanhagos 1/310; 2/248
 The reason: As Maariv is Tadir, and one may not precede a non-Tadir to a Tadir. [ibid]
 Piskeiy Teshuvos ibid footnote 16
 Rashal in Bava Kama 42 that so is custom of Israel; Darkei Chaim Veshalom 626; Kaf Hachaim 489/80; Igros Kodesh 19/272 [printed in Shulchan Menachem 3/1]; See Piskeiy Teshuvos 489/19
 Darkei Chaim Veshalom 626; Minchas Yitzchak 6/45 in name of Gedolei Hachassidus
 Taamei Haminhagim 613; See Piskeiy Teshuvos 489/19
 Admur 489/12; Michaber 489/3 as explained in Taz 489/6; M”A 489/7
 Admur ibid; Michaber ibid
 Admur ibid; M”A ibid
The reason this Tnaiy helps even according to those who rule the Mitzvos do not need intent: As even according to those opinions who hold that Mitzvos do not need intent to fulfill ones obligation, if one specifically has in mind to not fulfill his obligation, everyone agrees he is not Yotzei, as he cannot be Yotzei against his will. Therefore, if one remembers at night of his stipulation, he is to count with a blessing. [489/12; Rama 489/3; M”A 489/8; Chok Yaakov 489/14]
Other opinions: Some Poskim rule that making a Tnaiy is useless, as one cannot questionably join the fulfillment of a Mitzvah. [Taz 489/6] It is thus questionable whether he has fulfilled his obligation even if he made a Tnaiy and remembered later on. [P”M 489 M”Z 6]
 Admur ibid; M”A 489/8
The reason: As perhaps when he counted by Bein Hashmashos it was already night, in which case he fulfills his obligation according to those opinions who say that Mitzvos do not need intention for one to fulfill his obligation. Furthermore, even according to those Poskim who argue that Biblical Mitzvos require intention, today the Mitzvah of Sefira is Rabbinical, and there are Poskim who rule that by a Rabbinical Mitzvah according to all one does not need intention. [Admur ibid]
 As he has not fulfilled his obligation according to all opinions, as perhaps Bein Hashmashos is day, and perhaps we require intent to be Yotzei even by a Rabbinical command.