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One may not fast from the first of Sivan until after Shavuos. [This applies until, and including, the 12th of Sivan.]
Is a Chasan and Kallah who are getting married between the 1st and 12th of Sivan to fast on that day?
Some Poskim rule a Chasan and Kallah are to fast on such a day. Other Poskim however rule that they are not to fast on any day that Tachanun is omitted. In a case of need, such as a Chasan/Kallah who is very weak, one may be lenient.
 Admur 494:20; M”A 494:4; Peri Chadash 494; See Piskeiy Teshuvos 494:9
Other opinions: Some Poskim rule it is permitted to fast on the 2nd of Sivan. [Halef Lecha Shlomo 331]
 Based on Admur in Siddur regarding Tachanun; Piskeiy Teshuvos 494:9
Ruling of Shulchan Aruch Admur: In the Shulchan Aruch 494:20 Admur rules that one may fast beginning from the 9th of Sivan [in the Diaspora]. So also rules: M”A 494:4
 M”A 573:1; Chayeh Adam 132:43; Kitzur SHU”A 146:2; M”B 573:7
 Elya Raba 573:3 in name of Nachalas Shiva; M”B ibid in name of Yeish Makilim; Mishmeres Shalom 38:3; Ashel Avraham Butchach ibid
 Mateh Efraim 625:2; Daas Torah 573; Piskeiy Teshuvos 493:9