Halocho #195 What's BeHaB?

On the first "Monday, Thursday and Monday" in the month following Pessach and Sukkoth the custom is to say extra Selichot. Some communities do this on the last "Monday, Thursday and Monday" of the month. Since Monday is "Yom Sheni" or "Yom Bet - ב" and Thursday is "Yom Chamishi" or "Yom Heh - ה" these days are referred to as BeHaB - בה"ב Some people have the custom to fast on BeHaB. A special Mi-Sheberach was recited on Shabbat before the Torah was returned to the Aron Hakodesh to bless those who will fast. Despite being a "private" (not communal) fast day, a person need not "declare his intention to fast" during the Mincha-Amida of the preceding day, if he answered Amen to the Mi-Sheberach. Answering Amen to the Mi-Sheberach does not oblige one to fast. Even if one intended to fast when answering Amen and then on the designated day there was a Se'udat Mitzva (e.g. Brit Mila or Siyum or Pidyon HaBen) one should join the meal and not fast. If one "declared his intention to fast" during the Mincha-Amida of the preceding day, then one needs to fast. Tomorrow - Monday, and Thursday and next week Monday are BeHaB in most communities. Source: Kitzur Shulchan Aruch 127:3, 14 - Danny Sunday, 4 Marcheshvan 5769