For this reason no supplications are offered on any of the days of Nisan and there is no fasting until Nisan has passed. The only exception is [the fast of] the firstborn who fast on the eve of Passover. The very pious also fast on that day on account of the unleavened bread, in order that they shall begin to eat it with relish. Scholars fast on Mondays and Thursdays [throughout the year] on account of the desecration of the name of God and for the honour of the Temple which was burnt.
We follow the version in Tractate Sofrim: Firstborns fast on the eve of Passover. And the reason is to remember the miracle, that they were saved from the plague of the firstborn. And Avi HaEzri wrote that the firstborn from the father should also fast, just like the plague was also against them. However, the head of the household does not need to fast [if he is not a firstborn]. Even though the plague was [also] against them, we are not stringent to that degree. A delicate person should fast in order that he can eat matza with relish [that night]. And it is found like this in the Gemara (Pesachim 108a:2,5): Rav Sheshet would fast the entire eve of Passover[...] It was since he was delicate; if he would eat during the day, he would not be able to eat at night.
The firstborns, either of the father or of the mother, must fast on the eve of Passover. According to one opinion, even a firstborn daughter must fast (it is not the practice).
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Rebbi Levi said, he who eats mazzah on Passover Eve is like one who sleeps with his betrothed in his father-in-law’s house, and he who sleeps with his betrothed in his father-in-law’s house is flogged. It was stated: Rebbi Jehudah ben Bathyra says, both bread and mazzah are forbidden. Rebbi Simon in the name of Rebbi Joshua ben Levi: Rebbi ate neither bread nor mazzah. No mazzah because of that of Rebbi Levi, and no bread because of that of Rebbi Jehudah ben Bathyra. Was Rebbi a student of Rebbi Jehudah ben Bathyra? No, he was a student of Rebbi Jacob ben Qorshai. But it was because he was a firstborn. Rebbi Mana said, my father Rebbi Jonah was a firstborn and he ate. Rebbi Tanḥuma said, it is neither for this nor for that reason; Rebbi was asthenic and when he ate during the day, he could not eat in the night. Why did he not eat during this day? To enter the Sabbath with appetite.
The Gemara relates that Rav Sheshet would fast the entire eve of Passover. The Gemara asks: Shall we say that Rav Sheshet maintains that this practice was necessary because of two factors? First, when the mishna states that one may not eat adjacent to minḥa time, we learned this ruling with regard to the period of time adjacent to the greater minḥa, and the reason for the prohibition is due to the Paschal lamb, lest one be drawn after one’s meal and come to refrain from performing the sacrifice of the Paschal lamb.