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Prior to counting the Omer at night one must say a blessing, just as is done prior to the fulfillment of every Mitzvah, whether Biblical or Rabbinical. The blessing is “Baruch Ata Hashem Elokeinu Melech Haolam Asher Kidishanu Bemitzvosav Vetzivanu Al Sefiras Haomer.”
Counting during the day: If one forgot to count the Sefira at night and remembered during the day, he is to count the Sefira without a blessing. See Halacha 19! If one forgot to count at night and counted during the day without a blessing, as explained above, then he has not forfeited the blessing and may thus continue to count with a blessing on the following nights. See Halacha 20!
Counting after missing a day: In the event that one forgot to count one full day of Sefira, meaning that he forgot to count both at night and also the next day, then the custom is to continue to count on the following nights without a blessing. See Halacha 21!
Counted wrong date: If one counted the wrong date and only discovered the next night, then it is considered that he did not count that day and he may thus no longer continue counting with a blessing. See Halacha 21!
Fulfilling the blessing through hearing it from another person: See Halacha 16!
Why is a blessing recited over Sefiras HaOmer and not over other countings that we do, such as the counting for a Zava?
Some Poskim explain that this is because a Zava is not obligated to immerse in a Mikveh if she does not desire to do so, [and hence there is no Mitzvah involved in her counting]. Others however explain the reason is because it is possible that she will see blood during the seven days and hence not be able to immerse and fulfill the Mitzvah that she was counting for.
Why is the blessing of Shehechiyanu not recited on the first night, prior to counting Sefiras Haomer?
Some Poskim explain that Shehechiyanu is not recited prior to beginning the Sefira each year being that the Mitzvah does not contain any pleasure or action, and Shehechiyanu is only recited before a Mitzvah that one has benefit from and contains an action. Others explain that we do not recite Shehechiyanu because the Mitzvah [today] is in commemoration of the destruction of the Temple. Others explain that we do not recite Shehechiyanu because the time of Sefira is a time of judgment. Others explain that since the entire purpose of Sefiras Haomer is to purify ourselves in preparation for Matan Torah, and in truth we anticipate the ending of the count in order so we can receive the Torah, therefore it is not relevant to say Shehechiyanu on such a Mitzvah.
Is the congregation to be Yotzei their blessing with the blessing of the Chazzan, or are they to recite their own blessing individually?
See Halacha 16 in Q&A!
 Admur 489/3; Michaber 489/1
 Siddur Admur; See Sefer Sefiras Haomer p. 11 for sources of this dialect, in contrast to the dialect of :Yimei Haomer” or Lispor Haomer”
 See Admur 489/21 “As he said a lie, as today is the 4th day and he verbalized that it is the 5th day. Now, although he had in mind to say the 4th day, it is meaningless, as the Mitzvah of counting Sefira is fulfilled only with ones mouth.”
 According to all, if a woman is only a Niddah and not a Zava then she is not required to count at all, as she does not need to Biblically count seven clean days. Now, although today we require every woman to count seven clean days, even if she is not a Zava, this is due to Takanas Rebbe Yehuda, and the Minhag of Bnos Yisrael. [See Hakdama of Admur to Yoreh Deah 183]
 Radbaz 27, brought in Chok Yaakov 489/6
 This explains why according to the Radbaz we find other Mitzvos which are also not obligatory and yet a blessing is recited prior to their performance, such as by the actual immersion of the Zava or by Shechita. The reason is because when one decides to become pure he must immerse in a Mikveh, or when one desires to eat meat he must slaughter as the result of a positive command, and hence certainly a blessing should be said just as a blessing is said prior to any Mitzvah. However, the counting of a Zava is not the Mitzvah in it of itself and is rather the mere preparatory stage, similar to preparing an animal for the slaughter, and hence no blessing is recited, as there is no command to count the days but rather to immerse, and hence a blessing cannot be said. Furthermore, since she can count the seven days and then simply desire not to immerse, being she is not commanded to do so, therefore there is no Mitzvah involved in the counting, but rather only in the actual immersion.
 Tosafus Menachos 65b; Chok Yaakov 489/6; See Likkutei Sichos 31 p. 170 footnote 5
 She can’t even bless on the seventh day, towards evening, as perhaps she will see blood before she immerses. The Chok Yaakov ibid rejects the explanation of the Radbaz, as there are many Mitzvos that are not an obligation and we nevertheless say a blessing upon their fulfillment, such as the Mitzvah of Shechita, and other Mitzvos, and thus he concludes that the explanation above of Tosafus is the correct explanation.
 See Piskeiy Teshuvos 489/4
 Beir Heitiv 489/5 in name of Achronim
 Hamaor end of Pesachim, brought in Daas Torah 489
 Elya Zuta 493 in name of Rabbeinu Yerucham 1 5/4
 Kedushas Levi Parshas Nasso