There is a custom to observe the day of Erev Pesah as "Ta'anit Bechorim," the "Fast of the Firstborn," on which male firstborn are required to fast. The common practice is for the firstborn to participate in a Siyum (completion of a Talmudic tractate) which absolves them of the obligation to fast.
What is the reason underlying this custom? True, as we know, the firstborns of Bene Yisrael were spared from the divine plague that struck the Egyptian firstborn on the night of the Exodus from Egypt. Seemingly, however, this event should be a cause of celebration, rather than fasting. Why do the firstborn not conduct special festivities or recite Hallel on Erev Pesah? Why do they mark the event of Makat Bechorot (the plague of the firstborn) through a fast?
A number of different explanations have been offered for this custom.
Hacham Ben Sion Abba Shaul (Israel, 1924-1998), in his work Or Le'siyon (vol. 3, 12:1), suggested that the firstborn fast in an effort to compensate for the merits lost as a result of Makat Bechorot. The Sages teach that whenever a miracle is performed for a person, this event diminishes from his accrued merits. The plague of the firstborn, during which all Egyptians firstborns died while all the Israelite firstborns survived, was clearly a miraculous event which thus diminished from the merits of Bene Yisrael's firstborns. The custom is therefore for the firstborn to observe a fast in order "replenish" their merits.
A different explanation appears in the work Va'ya'an Yosef, which noted that the Torah speaks of Bene Yisrael's homes being "saved" during the plague of the firstborn ("Ve'et Batenu Hisil" – Shemot 12:27), clearly indicating that the firstborn had been under the threat of the plague. Evidently, the Midat Ha'din (Divine attribute of strict justice) was awakened against Bene Yisrael, and called for their inclusion in the deadly plague. Indeed, the Sages mention in other contexts that Bene Yisrael had worshipped idols in Egypt just as the Egyptians did, and were therefore, strictly speaking, deserving of the same fate as the Egyptians. The firstborns of Israel were thus initially threatened by the plague, and were saved only as a result of the Midat Ha'rahamim (Divine attribute of compassion) that overpowered the Midat Ha'din.
Accordingly, the Va'ya'an Yosef suggests, the firstborn fast even today in order to awaken the Midat Ha'rahamim and neutralize the effects of the Midat Ha'din. Each year on Erev Pesah, the Midat Ha'din that had arisen against Bene Yisrael in Egypt reawakens, and threatens to punish the firstborn among the Jewish people, Heaven forbid. This reawakening of the Midat Ha'din is similar to a Halacha established in Masechet Shabbat (129) warning against bloodletting on Erev Shavuot. On the day before Matan Torah, Bene Yisrael were under the threat of the Midat Ha'din due to the possibility that they would not accept the Torah, and this threat resurfaces on that date each and every year. One therefore should not enter into dangerous situations such as bloodletting at this time. Likewise, the Va'ya'an Yosef claims, the Midat Ha'din against the firstborn returns every year on Erev Pesah, and therefore the firstborn must fast in order to awaken the Midat Ha'rahamim.
A third explanation associates this custom with the famous tradition that the firstborn were to have served as the nation's Kohanim, and it was only as a result of the sin of the golden calf that this privilege was transferred to the tribe of Levi. It turns out that the firstborn served as Kohanim for the last time on the night of the Exodus, when Bene Yisrael offered the Korban Pesah (paschal sacrifice). This marked the last sacrificial ritual observed before the sin of the calf, and thus marked the last time the firstborn served as the Kohanim. The firstborn therefore fast each year on Erev Pesah in order to make a Tikun ("rectification") for their participation in the sin of the golden calf, as a result of which they were denied the privileges of the priesthood.
