Q) My new-born baby is having his bris on Ta’anis Esther. Am I supposed to fast?
A) The Shulchan Aruch in Hilchos Tisha B’av (OC 559:9) regarding a bris on Tisha B’av nidche rules that the ba’alei bris [i.e. the father of the baby, mohel and sandek] should not complete the ta’anis [i.e. they should stop fasting once the bris has been performed] because it is their personal Yom Tov.
The Taz (ibid s.k. 7) explains that this only applies to Tisha B’av nidche; but not to a regular Tisha B’av.
The conclusion of the Magen Avraham (s.k. 110) is that the same din applies to all ta’aneisim that are nidche; however, when a bris falls on a ta’anis that is not nidche then this dispensation does not apply, and the ba’alei bris are required to complete the fast.
The Mishna Berura (s.k. 35) rules in accordance with the Magen Avraham.
In a similar vein, regarding a bris that falls on Ta’anis Esther mukdam [i.e. when Purim falls on Sunday Ta’anis Esther is brought forward to the preceding Thursday] the Mishna Berura (686:7) rules that the ba’alei bris may eat on the ta’anis after the bris.
[It appears that the status of Ta’anis Esther mukdam is similar to the status of a ta’anis nidche; the common denominator being that the ta’anis is not observed on the fixed date when it coincides with Shabbos.]
The Vilna Gaon (on Shulchan Aruch OC Siman 686) opines that a ba’al bris may eat on a regular ta’anis – even when it is not nidche; and only regarding Tisha B’av do we find a distinction between when it is nidche or not – i.e. only on Tisha B’av nidche may ba’alei bris eat.
The conclusion of the Mishna Berura (Sha’ar Hatziyun 686:16) is that regarding a bris that falls on a ta’anis, a ba’al bris should only be lenient and break the fast when it is nidche; however, regarding Ta’anis Esther “that is not as obligatory as the other fasts” a ba’al bris may break his fast and eat after the bris – even when it is not nidche.