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From which day of the new month may one begin saying Kiddush Levana?
There are various opinions regarding how many days into the month must pass for Kiddush Levana to be said. Some Poskim rule one may say Kiddush Levana even on the first day of the moon’s visibility. Other Poskim rule one may only begin to say it after three days have passed from the Molad. Other Poskim rule one must wait until seven days pass from the Molad. [Based on Kabala, one is not to begin to say Kiddush Levana until seven days from the Molad have passed. Practically, the Chabad custom and the custom of many sects of Jewry, is to wait seven days from the Molad. Other sects of Jewry are accustomed to say it after the passing of three days, and in a time of need even prior to three days. Even according to the Chabad custom, in those areas that do not have much visibility of the moon due to cloudy skies, as is common in the winter months, if Motzei Shabbos falls after three days but prior to seven days, Kiddush Levana is to be recited that Motzei Shabbos and is not to be delayed. In cases of question or doubt in this matter one is to follow the ruling of the Rabbinical authority of his community.]
One should not say Kiddush Levana until seven days have passed from the Molad. However, if one suspects that the moon will not be visible after the seventh, then if Motzei Shabbos falls after three days but prior to seven days, Kiddush Levana is to be recited that Motzei Shabbos and is not to be delayed. In cases of question or doubt in this matter one is to follow the ruling of the Rabbinical authority of his community.
Must one wait seven complete days [i.e. Zayin Shleimim] from the Molad to say Kiddush Levana?
Some Poskim rule it is not necessary to wait until seven full days pass from the Molad, and rather as soon as six days have passed and one has now entered into the seventh day past the Molad, Kiddush Levana may be recited. Others however rule that based on Kabala one is to wait for a full seven days to pass from the Molad before saying Kiddush Levana. Practically, one may be lenient in this matter.
 Rambam; Smag; Rashal; Sefer Hakaneh, brought in M”A 426/13; Gr”a and other Achronim brought in M”B 426/20
 Rabbeinu Yona; Levush 426; Bach 426; Taz 426/3; M”A 426/13; Peri Chadash; Beir Heiytiv 426/10; Siddur Yaavetz; main opinion in M”B 426/20
Opinion of Taz 426/3: The Taz ibid brings the Bach who wonders at the above ruling that one must wait until seven days pass. The Bach rather rules, as rules Rabbeinu Yona, that after three days have passed one may say Kiddush Levana. The Bach concludes that practically the custom is to only wait three days, and hence if Motzei Shabbos falls on the fourth day or onwards Kiddush Levana is to be said and the Mitzvah is not to be delayed. The Taz ibid rules, as does the Levush, that if Motzei Shabbos falls after three days have passed then if the moon is already giving much light, Kiddush Levana is to be said, and one is not to delay until the seventh. [Hence the Levush and Taz conclude it is not enough for three days alone to pass but the moon must also visibly be giving much light.]
Opinion of M”A 426/13: The M”A ibid brings all three opinions regarding when one may start saying Kiddush Levana. He concludes [so is implied] like the Bach and if Motzei Shabbos falls within the first three days they are to delay until next Motzei Shabbos.
 The reason: As from three days after the Molad the moon is visible enough to benefit from it.
 Michaber 426/4 based on Kabalists brought in next footnote; Hagahos Maimanis brought in M”A 426/13
 Admur in Siddur and Peri Megadim brought in M”B 426/20; The Molad is the calculated time that the moon will begin its new cycle.
 Admur in Siddur based on Kabala; Michaber 426/4; Hagahos Maimanis brought in M”A 426/13; This is based on the Kabalist Rav Yosef Gegetilya, the author of Shaareiy Orah; So is also written in: Mishnes Chassidim Miseches Motzei Shabbos; Maggid Meisharim Shir Hashirim; See Hagahos of Rebbe Rashab on Siddur Im Dach p. 321 for other Kabalistic sources, and sources in Chassidus. See Nemukei Orach Chaim 426/4; The Shaar Hakolel 33/2 suggests that there is no source for this in the Kabala of the Arizal or from the Gemara. The Rebbe Rashab however in the glosses ibid negates this remark and brings sources in Kabala, and Chassidus for this practice.
The reason: As one can only sanctify the moon after it has received from the seven Middos.
 The Chabad Custom: As stated above, Admur in the Siddur writes that based on Kabala one is to wait seven days past the Molad. So is also written in Torah Oar [109b]. Practically, however, in the winter the Tzemach Tzedek would say Kiddush Levana even before seven days have passed from the Molad. The reason for this is because of the cloudy and rainy weather experienced in the Russian winter which prevents the moon from being seen. Hence, delaying Kiddush Levana when the opportunity arises prior to seven days could cause one to not say it at all due to the moon’s inability to be seen past the seventh. [Glosses of Rebbe Rashab in Siddur Im Dach p. 321; Shaar Hakolel 33/2; See there [in Shaar Hakolel] that he suggests the reason the Tzemach Tzedek did not wait until seven days is because he found no source in the Kabala of the Arizal or from the Gemara. The Rebbe Rashab however in the glosses ibid negates this remark and brings sources in Kabala, and Chassidus for this practice.]
 The majority of Achronim [Bach; Taz ibid; M”A ibid; Peri Chadash; Beir Heiytiv 426/10; main opinion in M”B 426/20] dispute the ruling of the Michaber and Kabala, and rather rule that if Motzei Shabbos falls three days past the Molad or onwards one may say Kiddush Levana on Motzei Shabbos even though it is prior to the seventh of the month. Many of Lithuanian Jewry follow this ruling
Other Chassidim: In his glosses of the Siddur [p. 360] Rav Raskin records a tradition of the dynasty of Ruzhin that the Baal Shem Tov was not particular to wait until seven days pass while the Maggid of Mezritch was particular to wait. This is one of the instances that the custom of the Maggid differed from that of his teacher the Baal Shem Tov. See Nemukei Orach Chaim 426/4
Opinion of M”B: The M”B ibid brings that some Achronim, including the Gr”a, rule that one may say Kiddush Levana even before three days have passed, and even prior to Motzei Shabbos. He concludes that one who follows this opinion has upon whom to rely, and in an area of cloudy weather he is praised for doing so.
 And hence delaying an opportunity of saying Kiddush Levana could cause one to not say it at all.
 Rebbe in Sichas Noach 5752, brought in Shaareiy Halacha Uminhag 2/179; See previous footnotes.
The Rebbe writes as follows: If one is not stringent one can say Kiddush Levana on Motzei Shabbos that falls after the 3rd day past the Molad, even though it is before the seventh. This especially applies in cloudy areas and particularly in the winter. The Rebbe concludes that: each community should follow in accordance to his level of cloudiness, and it is given to the discretion of the local Rav to decide whether to precede the Kiddush Levana to Motzei Shabbos that is before the seventh of the month. It is implied from the above wording that [even according to the Chabad custom] one may choose to say Kiddush Levana on Motzei Shabbos that is prior to the seventh even if it is not cloudy or rainy. When it is cloudy or rainy one is specifically to do so. However from footnote 14 there it is implied that in a non-cloudy area one is to wait.
 Rameh Mepuno 78; Shiyurei Kneses Hagedola; Elya Raba 426/14 [brought in Shaareiy Teshuvah 426/10] that one may say Kiddush Levana on the night of the 7th; Ashel Avraham Butchach 426/2
 Birkeiy Yosef 426/4 brought in Shaareiy Teshuvah 426/10; Mateh Efraim 581/10 rules one is to initially say it only after seven complete days have passed, although if he suspects the moon will not be visible again, he is not to delay the blessing.
 So concludes Hiskashrus 454 p. 15 and so was the custom of Rav Yaakov Landau [as told to me by his son Rav Eliyahu Landa Shlita].
Seemingly the reason one may be lenient is because even regarding waiting seven days there is a dispute and the Rebbe leaves room to be lenient. See also Sichas 1986 6th Adar Rishon that Kiddush Levana was said that Motzei Shabbos even though seven full days had not passed since the Molad.