We find mention of this special fast in Shulchan Aruch Orech Chayim 492. The Shulchan Aruch writes "There are those that have a custom to fast the Monday, Thursday and Monday after Pesach and also after Sukkos, and we wait until after the entire month of Nissan and Tishrei passes before we fast." The Mishna Berura explains that the reason for the fast is similar to that which we said above: because of the festivities and happiness on the holiday, we might have come to sin, and therefore there is need for repentance. The reason why we wait until after Tishrei and Nissan to fast, the Mishna Berura explains, is because both months are "festive" months, and therefore it is not proper to dampen the spirit of the months with fasts.

Nowadays, not everyone fasts. There are those who have a custom not to fast, but to still say Selichos, the penitential prayers which are normally recited on fast days. This year, the Fasts of B'hab (which is a contraction of the Hebrew letters Bais Heh Bais - the letters which stand for Monday and Thursday) are on Monday, October 30; Thursday, November 2; and Monday, November 6.

In Ta'amei HaMinhagim, two other reasons are given for why we fast at these times. The reason given by the Levush is that since the seasons and the air changes at these times, there is a greater chance of a person getting ill. We therefore fast so that people should not become ill at this time. The reason given by the Sefer Chasidim is that these times are crucial periods for farmers - after Pesach, heat should not damage crops, and in the winter after Sukkot, there should be rains and winds which have a good effect on the crops. We therefore fast at these times so that the crops and the farmers should meet only with success.