Hilchos Sefiras Ha’omer – 11

1. Some have a custom to fast on the Monday, Thursday and second Monday following Pesach, but they begin the series of fasts only after the conclusion of the month of Nissan – a month in which we do not fast (Shulchan Aruch and Mishnah Berurah 492).

2. The reason for these fasts is that we are concerned that perhaps in the course of the feasting and rejoicing during the festival people may have come to sin. We fast as an atonement for those possible sins (ibid.).