Omitting Tachanun during the month of Sivan:
In The month of Sivan, Tachanun is omitted from the first day of Sivan until the 12th of Sivan, including on the 12th of Sivan. The following are the details of this custom.
A. Omitting Tachanun from Rosh Chodesh Sivan until after Shavuos:
The custom is to omit the reciting of Tachanun from the beginning of Sivan until the 8th of Sivan. The reason for this is because starting immediately after Rosh Chodesh Sivan, Moshe began preparing the Jewish people for the receiving of the Torah [and thus every day from Rosh Chodesh Sivan until the 8th of Sivan contains an event that makes it considered a times of joy, when Tachanun is to be omitted].
The following are the daily preparations that took place:
- First of Sivan: On Monday, the first of Sivan, is Rosh Chodesh and hence Tachanun was omitted.
- Second of Sivan: On Tuesday, the 2nd of Sivan, Moshe told the Jewish people “You will be for me a Mamleches Kohanim.”
- Third of Sivan: On Wednesday, the 3rd of Sivan, Moshe told the Jewish people the Mitzvah of Hagbala, to not enter into the vicinity of the mountain during Matan Torah.
- Fourth and Fifth of Sivan: On Thursday, the 4th of Sivan, Moshe told the Jewish people the Mitzvah of separating from their wives for the two days of Thursday and Friday and that they should be prepared for the 3rd day, which is Shabbos on which the Torah was given.
- 6th and 7th day: Is Shavuos.
- 8th day: Is Motzei Yom Tov which is called Isru Chag.
B. After Shavuos, until the 13th of Sivan:
Some Poskim rule that that one resumes to recite Tachanun on the 9th of Sivan. Other Poskim, however, rule that one is to omit Tachanun until the 12th of Sivan, including the 12th of Sivan, [and so is the Chabad custom]. [Tachanun is resumed on the 13th of Sivan. On the Shabbos of these dates, Av Harachamim and Tzidkascha Tzedek is omitted.]
 See Admur 494:20; Siddur Admur; Rama 131:7; 494:3; Kaf Hachaim 494:52; Piskeiy Teshuvos 494:9; Nitei Gavriel p. 221
 Admur 494:20; Siddur Admur; Rama 131:7; 494:3; Hagahos Maimanis Tefila 5 Shin; Minhagei Tirna Minhagei Sivan
 Admur ibid; M”A 494:4; Kaf Hachaim 494:50
 Admur ibid
 Shemos 19:6
 Shemos 19:12
 Shemos 19:10-11
 Custom in Eretz Yisrael: According to the Shulchan Aruch, being that in Eretz Yisrael there is only one day of Yom Tov, therefore Tachanun should resume in Eretz Yisrael on the 8th day of Sivan. However, according to the Siddur, we omit Tachanun until the 13th. Practically, the custom in Eretz Yisrael is to omit Tachanun until the 12th of Sivan. [Piskeiy Teshuvos 494:9; Nitei Gavriel p. 221]
 Admur 494:20; Rama 131:7; 494:3; Levush 494.
 Siddur Admur; M”A 131:18; Kneses Hagedola 494:6; Peri Chadash; Elya Raba 494:11; Shulchan Gavoa 494:8; Moed Lekol Chaiy 8:44; Yifei Laleiv 2:6; Shulchan Hatahor 131; P”M 131 A”A 18; Shaareiy Teshuvah 131:7; M”B 494:36 in name of M”A and P”M; Kaf Hachaim 494:52
 The reason: The reason for this is because after Shavuos there is 7 days of Tashlumin for which the Holiday Karban is able to be sacrificed, and therefore we do not say Tachanun until the days of Miluim of the Karban have passed.
Other opinions: Some Poskim rule that one is to omit Tachanun until the 13th or 14th of Sivan. [See Kneses Hagedola ibid; P”M ibid; Kaf Hachaim ibid; Shaareiy Teshuvah ibid]
 Siddur Admur; Likkutei Sichos 28:76 [printed in Shulchan Menachem 3:42]
Other opinions: Some Poskim rule Tachanun is to be omitted in the Diaspora likewise on the 13th of Sivan due to Sfeika Deyoma. [See Shaareiy Teshuvah ibid; See Kneses Hagedola ibid; P”M ibid; Kaf Hachaim ibid; Shaareiy Teshuvah ibid]