Erev Pesach is a fast day for male firstborn. The prevalent custom nowadays is for bechorim to participate in a siyum, which permits them to eat the rest of the day. However, we do not find this principle – that a siyum permits one to eat on a fast day – at work anywhere else in Jewish law. Why is the fast of the firstborn different?
During the Nine Days, a person may make a siyum in order to eat meat or drink wine. But he may only do so during the seudas mitzvah. In contrast, a siyum on the fast of the firstborn permits one to eat the entire day. Why? Why do we treat it with less seriousness?
The reason for the fast day altogether is unclear. The Tur (Orach Chayim 470) explains that the fast commemorates the miracle that during makkas bechoros only Egyptian firstborn died while Jewish firstborn were spared.
Mefarshim raise several questions with this explanation. First, the makka occurred on the first night of Pesach, not on Erev Pesach. Why, then, fast on Erev Pesach? Additionally, we generally celebrate salvation with a holiday, not a fast. Why is this salvation celebrated with a fast day?
The Aruch Hashulchan (Orach Chayim 470:5) writes that for generations bechorim have been lenient with this fast. He writes that he does not know, however, where this leniency originated. He suggests that it was probably a result of the weakness of our generations and the hard work that we do on Erev Pesach. Furthermore, it is difficult to eat marror after fasting a whole day.
There are also questions regarding who is supposed to fast on this day. Is it only the bechor of a father? How about the bechor of a mother? How about the eldest in the house (who, in Mitzrayim, died during makkas bechoros)? What about female firstborn? The Shulchan Aruch states that both the bechor of a father and of a mother should fast, and quotes an opinion that even female firstborn should fast as well. The Rema states that the minhag is only for male firstborn to fast.
Some suggest that the remembrance of the miracle of yetziyas Mitzrayim begins with “Passover” – the miracle of Hashem passing over the houses of the Jews and killing the Egyptians. It was this act that sanctified the bechorim, as the Torah says, “For every firstborn is Mine; on the day I struck down every firstborn in the land of Egypt, I sanctified every firstborn in Israel for Myself” (Bamidbar 3:13).
The Gemara (Pesachim 64a) says that Erev Pesach traditionally was always the busiest day in the Beis Hamikdash. Therefore, this day was always the most difficult for the bechorim as they felt their loss (since they were originally supposed to serve instead of the kohanim) more than ever. This is specifically why, according to some, only male firstborn of a father fast since they were the ones who would have performed the avodah had they not been later replaced by the kohanim.
So, in other words, the fast doesn’t commemorate the miracle that Jewish firstborn were saved; it commemorates the fact that they were sanctified on this night (some point out that Yaakov Avinu acquired the bechorah from Esav on Erev Pesach too), and they fast because of the distress they feel at having lost the opportunity to serve Hashem as they were originally supposed to. It is a day to fast, daven, and do teshuvah.
Some explain that we allow a bechor to eat once he makes a siyum, specifically on this fast day, because the Rambam says there are three crowns in Klal Yisrael; kehuna, malchus, and Torah, which is the highest level. Bechorim lost their kehuna crown, but they replace it with that of Torah. Hence, the siyum is not a “loophole” to enable eating on the taanis; rather, it serves as a substitute fulfillment of the taanis. With this explanation we can also understand why this seudah allows one to eat the entire day while a siyum during the Nine Days only allows one to eat meat or drink wine during the seudas mitzvah itself.