Behab, or בה״ב, is an a series of three fasts which were instituted after the holidays of Pesach and Sukkos to atone for any possible frivolous behavior which may have occurred. In Shaare Hatikvah, we mark these days with the recitation of Selichos in the Chazzan's Repetition of Shacharis.
These fasts occur twice a year on the following dates:
After Sukkos: the Monday, Thursday and Monday following the 17th of Marcheshvan
After Pesach: the Monday, Thursday and Monday following Rosh Chodesh Iyar