Until what day of the month may Kiddush Levana be said?
Kiddush Levana may only be recited within the first half of the month. A lunar month consists of 29 days, 12 hours and 793 Chalakim. Thus, it may only be said up to 14 days 18 hours and 396.5 chalakim [i.e. 22 minutes] past the Molad. [However, there are opinions who allow it to be said until 15 complete days past the Molad. Other opinions allow in a time of need to say Kiddush Levana with a blessing up until the night of the 16th of the month, including the night of the 16th. Others rule it may even initially be said in a time of need up until 15 days 12 hours and 22 minutes from the Molad, which is 18 hours past the time of the first opinion. Others suggest it may be said even past this time [up until the end of the month]. Others allow saying it past this time [up until the end of the month] if one says it from the Tur or Gemara. Practically, one should not be lenient to say Kiddush Levana with a blessing past the first half of the month, and is to rather contact a Rav. In all cases one may read it from the Tur or Gemara.]
One is to beware to recite Kiddush Levana prior to 14 days 18 hours and 396.5 chalakim past the Molad. If it is past the first half of the month, one is to contact a Rav.
Story of Rebbe Rashab from memoirs of Rav Yaakov Landau-Free Translation
Date: Motzei Shabbos Beshalach 18th Shvat,
A number of members of Anash, myself included, had not recited Kiddush Levana, and on Friday night the moon became visible. I entered into the Rebbe Rashab during the Shabbos Friday night meal, after the Chazara on the Mamar “Az Yashir”, in order to ask his opinion whether to say Kiddush Levana or not.
The Rebbe Rashab replied to me: There is a lengthy discussion on this matter. Reb Dovid Openhaimer has a lengthy response on this, and he concludes that one is to say Kiddush Levana [on Shabbos]. However the Shvus Yaakov rules one is not to say Kiddush Levana. Reb Dovid Openhaimer discusses these points and he says that there are no Techumin [above ten Tefachim] and that all the prayers are elevated. The Shvus Yaakov however negates this.
Reb Yaakov Landau then asked: The above discussion [mentioned in the Shvus Yaakov and Rav Dovid Openhaimer] is only with regards to saying Kiddush Levana on Shabbos, however today [on the 17th of Shvat] it is also after the time.
The Rebbe Rashab replied: Tonight is already passed the time of Kiddush Levana. (The Rebbe then began calculating the amount of time that had passed since the Molad and the last time for Kiddush Levana had been on Thursday night.) The Rebbe then concluded “if so then it is better to say it tomorrow than to say it tonight.” It once occurred by my father [The Rebbe Maharash] that they recited Kiddush Levana 17 days after the Molad. However on Shabbos I never saw him say Kiddush Levana. This occurred in Lubavitch. After my father [the Rebbe Maharash] passed away it once occurred that the congregation recited Kiddush Levana on the second night of Sukkos. I said Kiddush Levana earlier. I saw the moon earlier so I already said the Bracha, however the rest of the congregation said it on the second night of Sukkos. (The Rebbe then mentioned to me a certain Rabbi that ruled they should say Kiddush Levana then.) Midmuvsky then reminded me that Reb Yisrael Noach once said Kiddush Levana on the second night of Pesach. Based on this one can also say Kiddush Levana on Shabbos as it is the same reason. Nevertheless I never witnessed anyone saying Kiddush Levana on Shabbos and in truth it is a bit difficult to accept such a practice. In my father’s time there were those that said Kiddush Levana after 17 days passed from the Molad. They awoke before daybreak and said Kiddush Levana. My father was always accustomed to awaken before daybreak however us they had to especially awaken for this occasion. That entire night someone stood guard outside to see if the moon would be seen. Feivish was standing guard. Practically, it is better [for Rav Landau] to say Kiddush Levana tomorrow night than to say it tonight. Tomorrow is the 17th day past the Molad. The Molad was at 12:00, thus you can say it tomorrow. Regarding saying Kiddush Levana on the 16th such an opinion is recorded in the Heishiv Moshe, however this was seemingly the occurrence with my father [the Rebbe Maharash].
The Rebbetzin Shterna Sara stated: I remember that at that night they all went outside with Sefarim.
The Rebbe Rashab replied: This occurred very late at night.
Rav Yaakov Landau asked: So it was said without the loophole of using a Gemara or Rif.
The Rebbe Rashab replied: Without using any loopholes. To do loopholes you don’t need to wait for the date to be the 17th, you can do it anytime.
Story of the Tzemach Tzedek and Reb Hillel Paritcher
During a certain snowy winter month Reb Hillel visited the city of Lubavitch, the town of residence of the Tzemach Tzedek. The moon had yet to be seen during that month and the last possible day to say Kiddush Levana had arrived. Reb Hillel sent a Pan to the Tzemach Tzedek to receive a blessing that the moon should be seen that night. The Tzemach Tzedek guaranteed Reb Hillel that the moon would indeed appear. Reb Hillel proceeded to station people outside as guard to announce to him when the moon becomes visible. In the midst of the night word came to Reb Hillel that the moon has appeared. Upon going outside he saw that the moon was a bit blurry covered by light clouds. He said that the Tzemach Tzedek said the moon would be seen, and therefore it has to be seen in a clear fashion, so he returned back inside awaiting for a clearer appearance. Slightly prior to daybreak the moon appeared clearly without any clouds interfering. Reb Hillel immediately went outside and recited Kiddush Levana. He then remarked that it once happened in his younger age that an entire month passed without a visible moon, although back then he was strong enough to handle such an occurrence. However, now in his old age he does not know how he would have been able to react if the moon had not been seen.
What has precedence, Davening Maariv or reciting Kiddush Levana?
One is to Daven Maariv prior to reciting Kiddush Levana, as Maariv is a more common Mitzvah and hence receives precedence. If however there is suspicion that one will not be able to say Kiddush Levana afterwards that month due to cloudy skies, then one is to say Kiddush Levana prior to Maariv.
What comes first, Havdala in Shul [for those Shuls which say Havdala] or Kiddush Levana?
May one recite Kiddush Levana prior to Megillah reading on Purim night?
If a congregation has not yet recited Kiddush Levana, they are to do so prior to Megillah reading.
If the moon became visible during Maariv may one stop in middle to say Kiddush Levana?
If one will certainly be unable to say Kiddush Levana after Maariv, such as that by then the time for Kiddush Levana will have expired, then he may stop in middle of Shema or Birchas Shema and recite Kiddush Levana. [In such a case he may only recite the actual blessing and not any of the other verses prior to or post the blessing.]
If the moon became visible during the reading of the Megillah on Purim may one stop in middle to say Kiddush Levana?
If the congregation did not yet recite Kiddush Levana and will certainly be unable to say Kiddush Levana after the reading, such as that by then the time for Kiddush Levana will have expired, then the congregation may stop in middle of the Megillah reading and recite Kiddush Levana. However, if the congregation already recited Kiddush Levana and there is an individual that did not yet recite it then since they will continue the reading while he goes outside to recite it, therefore he is to remain with the congregation for the reading of Megillah even if this will cause him to miss Kiddush Levana.
May one say Kiddush Levana if there is a Lunar Eclipse?
A lunar eclipse can only occur when the moon is full. This is a clear sign that the first half of the month has passed. In such a case one may no longer say Kiddush Levana even if in accordance to our calculation there is time still remaining.
 Rama 426/3; Bach; Shlah; Chayeh Adam 118/14; Kitzur SHU”A 97/10; Kaf Hachaim 426/53
Ruling of Michaber: Kiddush Levana may be said up to 15 days past the Molad. Once the 16th day past the Molad has begun it may no longer be said. [ibid] These days are calculated as 24 hour days and not by weekdays. [M”A 426/12; M”B 426/17] Thus according to the Michaber one has 6 more hours to say Kiddush Levana past the time of the Rama.
Ruling of Kaf Hachaim for Sefardim: The Kaf Hachaim 426/53 concludes that one is to follow the opinion of the Rama in these matters, as Safek Brachos Lehakeil, even against the Michaber.
Ruling of M”B: The M”B rules one may be lenient like Michaber. [Biur Halacha “Velo Tes Zayin Bechlal”]
 There are 1080 Chalakim in an hour. [Tur 427]
 The month begins at the time of the Molad of that month. Hence, one can say Kiddush Levana until the above time passes from the Molad and he is not to calculate it from Rosh Chodesh. [Michaber ibid]
Other Opinions: Some write one counts the above time not from the Molad but from Rosh Chodesh. [Taamei Haminhagim p. 200; See M”A 426/12]
 Michaber ibid; Mor Uketzia, and so concludes Biur Halacha “Velo Tes Zayin Bechlal”
 Kneses Hagedola in name of Rabbeinu Peretz; Heishiv Moshe 14; Tzeror Chesed; Shoel Umeishiv Kama 3/151; brought in Chasam Sofer 102 [brought in Alef Lamagen 581/22]; The Rebbe in 1961 15th Sivan said Kiddush Levana 1.5 hours past the time. The Rebbe stated he relies on the Chasam Sofer ibid; Rabbi Mordechai Perlow states that he remembers that a number of times they said Kiddush Levana in Lubavitch on the night of the 16th from a Gemara Sanhedrin
 Chasam Sofer ibid that it may even initially be said up until 18 hours past the time of the Rama
 The reason: As it takes 18 hours for the effects of the moon to be seen on earth. Thus the diminishing of the moon is not visible to earth until this time. [Chasam Sofer ibid]
 Suggestion in Chasam Sofer 102; Directive of Rebbe Maharash and Rebbe Rashab in Shemuos Vesipurim p. 184
The Chasam Sofer 102 defends saying Kiddush Levana with a blessing even within the second half of the month if one receives much joy from this, such as when the moon had not been seen the entire month and one looks forward to seeing it as a good omen. [See Teshuvah there for the exact case of allowance of Chasam Sofer] His reasoning is because it is a blessing of praise to Hashem for the joy and benefit of the moonlight. Nevertheless, he does not permit it unequivocally as is understood there from the Teshuvah. From the testimony of Reb Yaakov Landau recorded below it is evident that the Rebbe Maharash and Rebbe Rashab allowed saying Kiddush Levana even past the 16th of the month, and even without using a Gemara. The Rebbe Rashab once said Kiddush Levana 17 days past the molad. It was said with a blessing and not from a Gemara. [Memoirs of Rav Yaakov Landau, brought in Shemuos Vesipurim p. 184 see below]
 Divrei Torah 4/107; Darkei Chaim Veshalom 495; See also Avnei Nezer 383; Taamei Haminhagim p. 199 brings from Derech Pekudecha in name of the Chozeh Melublin that after the time has passed one may nevertheless read the blessing from the Gemara. Alef Lamagen 581/22 concludes from Derech Pekudecha that so is the custom of the world to say it from a Gemara with a blessing after the correct time; Mentioned in words of Rebbe Rashab in Shemuos Vesipurim p. 184; See Sheilas Yaavetz 81 that one may mentyion Hashem’s name when reading a blessing in the Talmud; Many Poskim however forbid doing so. [See M”A 215/5; Birkeiy Yosef 215 that so is custom of elderly Rabbis; Machazik Bracha in name of Tashbatz Katan 419 in name of Maharam, brought in Shaareiy Teshuvah 215/4; P”M 215 A”A 5; M”B 215/14]
 Gemara Sanhedrin 42a
 Hiskashrus 961 writes r [Vetzaruch Iyun as to his source] that one may say Kiddush Levana up to 18 hours past the time of the Rama [15 days 12 hours and 22 minutes from the Molad] as rules the Chasam Sofer, however he may not say it anymore once 18 hours have passed. In correspondence with Rav Ginzberg he wrote to me that this was the ruling of Rav Yaakov Landau, as was told to him by his son Rav Eliyahu Landau, that after 18 hours of the time of the Rama one may not be lenient to say it. This is despite the fact that the story mentioned above took place with Rav Yaakov Landau and he received a directive from the Rebbe Rashab to say Kiddush Levana on the night of the 20th. This story had been authenticated by the Rebbe Rayatz and was passed over his holy eyes prior to its publishing. Nevertheless, in actuality we are not lenient to rule this way. In an earlier volume of Hiskashrus  as well as in the glosses of Rav Raskin on the Siddur they wrote one may rely on the above story and recite Kiddush Levana even past the above time.
 Poskim stated in previous footnotes, see there. See glosses of Rav Raskin on Siddur who writes he recalls a directive of the Rebbe to say it from the Gemara when it is being said past the time.
 Brought in Shemuos Vesippurim p. 182; This story was reviewed and authenticated by the Rebbe Rayatz. [See Kfar Chabad 986]
 This story took place in 1919, of which Motzei Shabbos was the 18th of Shevat. The original mistakenly states the 19th of Shevat.
 The story was written by Rav Yaakov Landau.
 Shemuos Vesipurim Vol. 2 p. 57
 Machazikei Bracha 426/7; Shaareiy Teshuvah 426/9; Nodah Beyehuda 41; See Admur 431/6; See Teshuvos Vehanhagos 1/310; 2/248
and Piskeiy Teshuvah 489 footnote 16 in name of Shagas Aryeh and Rav Chaim Volozhin that they Davvened Maariv before Kiddush Levana even at a time that there was chance they would lose the opportunity.
 See Piskeiy Teshuvos 295/6
 Sefer Haminhagim p. 126 [English] regarding Motzei Yom Kippur; Likkut Dinei Rosh Chodesh 12/10
The reason: Perhaps this is because Havdala is more Tadir. [see Poskim in 681 regarding Chanukah]
 Nodah Beyehuda 41
 The reason: As Kiddush Levana is a more common Mitzvah, and possibly it will not be visible afterwards. [Noda Beyehuda ibid]
 Nodeh Beyehuda 41; Machazikei Bracha 426/3; Shaareiy Teshuvah 426/9
 Biur Halacha 426 “Velo Tes Zayin Bechlal”
 Nodeh Beyehuda 41; Machazikei Bracha 426/4;
 Maharil 19 brought in Beis Yosef, Kaf Hachaim 426/60