887. Fasting BH”B (Behab)

127:14 Regarding the fasts of the Monday, Thursday and Monday that are after Pesach and Succos (BH”B or “behab”), and also fasting during the ten days of repentance when one did not commit to do so at mincha, one might fast simply because doing so is the custom. In such a case, even if he intended to fast when he answered Amen to the Mi Shebeirach, since he did not commit to it at mincha, then if a bris, a pidyon haben, or another meal celebrating a mitzvah occurred on that day, it is a mitzvah for him to eat and he does not need to annul his obligation to fast (and he need not make up the day – Mishnah Brurah 568:17). Everyone who fasts on these days knows that it is the custom to fast but there is no custom to fast in lieu of participating in a festive meal. (If one committed at mincha to fast, then he is required to fast – Rema 568:2.)

127:15 If one becomes permitted to eat because of the meal of a mitzvah, he can stop fasting altogether and may eat even afterwards at home. Before the meal, however, it is forbidden to eat, except for the father of the baby and the sandek on the day of a bris. These may eat even before the meal because the day is like a yom tov to them. (Mishnah Brurah 568:18 extends this privilege to all the “baalei habris,” which would presumably include the mother and the mohel as well. He cites an opinion, however, that only permits the baalei habris to eat after mincha.)