Behab (בה"ב) is comprised of three Hebrew letters: bet – the second letter of the alphabet; hey – the fifth letter; and again bet. The sequence stands for Monday, Thursday and Monday. (As the Jewish week begins on Sunday, the morrow of the Shabbat, Monday is the second day of the week and Thursday the fifth.)

There is an ancient Ashkenazi custom to fast on the first Monday, Thursday and then the following Monday of the Jewish months of Cheshvan and Iyar—shortly following the Sukkot and Passover holidays.

These fasts serve to atone for any inadvertent sin which may have been committed as a result of over-the-top lightheadedness and merriment during the week-long holidays, when people are off of work for an extended period of time.

(Due to the special nature of the months of Tishrei and Nissan, these fasts are postponed until the following months. There is no fasting following the festival of Shavuot, because it is a short two-day holiday, leaving little time to engage in levity.)

Some find support for this custom in the book of Job (1:5): "Now it would come about when the cycle of the feasting days would be over, that Job would . . . offer up burnt-offerings . . . for Job said, 'Perhaps my sons have sinned and blasphemed G‑d in their hearts.'"

Today it is relatively rare to find people who follow this custom. Click here to read why penitential fasting is no longer the norm.

I hope that I've been helpful today.

Yours truly,

Rabbi Menachem Posner

Sources:
Tur, Orach Chaim 492; Turei Zahav and Magen Avraham on Orach Chaim ibid.