The Shulchan Aruch brings the minhag to fast on the Monday, Thursday, and Monday following Pesach (and Sukkos). The Mishnah Brurah cites the Mogen Avrohom that we fast because there is a serious concern that the excessive eating and drinking throughout the Yom Tov may have led to aveiros.
Why is there a greater concern of sin following a Yom Tov? Didn’t we just complete two nights of the seder strengthening our emunah and sanctifying ourselves with all the mitzvos of the chag, and partaking in mitzvah-ordained eating, including the mitzvos of achilas matzoh and seudos Yom Tov?
The Alter from Kelm asked: There are many aveiros that people commit and Chazal did not enact a fast day. Why is the possibility of sin brought on by the Yom Tov more of a cause to fast?
The Alter explains with a moshol. If one commits an act of robbery, it is self-understood that it is wrong and unforgiveable. The evil of robbery is common sense. If robbery would be allowed there would be no civility in the world and people would not be able to coexist.
However, there is an act of robbery that all would be in agreement is multiple times worse than an ordinary act of robbery. Imagine a homeless person, starving, unkempt, repulsive, and in dire need of a bath. Along comes a kind-hearted person and opens his home and heart to him. He overcomes his disgust, bathes him and feeds him, tends to all his needs and provides him with a comfortable bed. Every meal served is set royally with expensive dishes and elaborate finery.
In the middle of the night, this guest loads up a sack full of all the valuables used to serve him and flees. This may definitely be classified as an act of robbery. However, it is obvious that this was much worse. To do so to one’s benefactor, who treated and cared for him royally, is downright horrifying. A man who raised him from his guttural stench to humanity and respectability, to be maltreated in such a manner is outrageous and not merely an act of robbery.
Although the act of robbery was identical to any other robbery, the circumstances totally alter the perspective of the crime.
The Alter of Kelm continues. Definitely any aveirah transgressed is not to be taken lightly. When Hashem gifts us with a Yom Tov to sanctify ourselves, to become elevated and to deepen our connection to Him, it must be regarded as momentous and sacred. A ‘moed’ is a time of ‘hisva’adus’- of convening individually with the Eibershter. Yomim Tovim present us with a special aura and hashpaos that are limited to these occasions. If in these special moments that are to be utilized for growth, one indulges inappropriately, the aveirah is graver than if committed at any other time. For this, Chazal invoked the fast of B’hav.