Fast of the firstborn
הבכורות מתענין בערב פסח בין בכור מאב בין בכור מאם ויש מי שאומר שאפילו נקבה בכורה מתענה: (ואין המנהג כן)
The firstborns fast on the day before Pesach, whether they are the firstborn of their father or firstborn of their mother. Some say even firstborn women fast (Gloss: but this isn’t the custom)
There’s an ancient custom for the firstborn to fast on Erev Pesach, the day before Pesach. The common explanation for this fast day is that it’s in commemoration of The Plague of the First Born. The last of the Ten Plagues, all the firstborn Egyptians died at midnight. All the firstborn of the Jews were miraculously saved, so every year right before Pesach the firstborn fast. This sounds like it’s due to the gratitude of the firstborns that they fast.
However, this explanation at first glance is very hard to fathom. If there’s a desire to express gratitude for a miraculous salvation, why are the firstborn fasting? Every other similar situation in the calendar involves feasting and rejoicing, not fasting. This is the only fast that expresses gratitude. Furthermore, why should the firstborn of today be grateful? They have nothing to do with the original firstborn Jews who were saved. Only their direct descendants should have to fast, and who says someone who is today a firstborn is one of their descendants?
Perhaps another explanation can be suggested. There’s an interesting mitzvah that occurs the day before Pesach, on the fourth year of the seven-year Shemittah cycle. It’s called Vidui Ma’aser, literally the confession of the tithes. The mitzvah involves removing all tithes from one’s possession, by giving certain ones to the Kohen or the Levi, and by consuming some in the proper manner. Afterwards the person announces that they fulfilled all the laws properly. Chazal describe this as a confession. Why is so? It would seemingly be better labeled a declaration of fulfillment.
One suggestion is that this mitzvah is an implied rebuke. The task to ensure the Kohanim and Leviim have all of their tithes is something that didn’t have to be. Originally, the firstborn had the status of Kohanim. They were Hashem’s spiritual agents in this world. They would have been able to keep all of their tithes for themselves. Because, as we shall see, the Jews sinned. As a result, the firstborn lost their lofty status. Now only those who descend from Levi or Aharon can receive tithes. By declaring that we successfully cleared our houses of tithes, we are in effect confessing that our firstborn lost this status. This is why it is called Vidui Ma’aser.
What’s significant about specifically this mitzvah that it recalls the sins which caused the firstborn to lose their status? The date it occurs on. Erev Pesach is the busiest time of year in the Holy Temple. Millions of Jews converge to Jerusalem to partake of the Pesach offering. They bring their lamb to the Temple where the Kohanim perform the Temple service. They’re running all over the place, bringing this instrument there and that receptacle here. There’s a lot of work to do, and not a lot of time. Erev Pesach is where it is most pronounced that the Kohanim are Hashem’s holy agents, and not the firstborn. They work in full force, in view of the entire nation. It’s no wonder then that this is the proper time to reflect on why the Kohanim work in the Temple and not the firstborn.
When Hashem spared the firstborn from the final plague in Egypt, He sanctified them for His service. What caused them to lose this status? The sin of the Golden Calf. The Jews worshipped a false god, and as a result everyone’s firstborn son became like every other Jew. Only the tribe of Levi refrained from this idol worship, and were then granted this special status. In fact, the firstborn were the most zealous of worshippers of the Golden Calf. It makes sense then that it was specifically them who had an everlasting punishment.
It’s very reasonable then to suggest that the firstborn would fast every Erev Pesach. Not due to gratitude, but in mourning. They realized what they had lost. Every firstborn from then until now could have had a special status. Had there been a Temple, they would have performed the Temple Service. They would have received tithes from the nation. Erev Pesach is when their loss is most pronounced. They could have had much more, had they not sinned with the Golden Calf. Even though today we don’t have the Temple Service on Erev Pesach, the custom to fast never stopped. Perhaps this is why the firstborn fast even until day.
Good Shabbos and Chag Sameach!
 Based on Be’er Yosef Inyanei Pesach BeInyan Ta’anis Bechorim B’Erev Pesach
 Shulchan Aruch Orach Chaim 470:1
 The Tur ad. loc. brings it from Soferim 21:3, brought by Tosafos to Pesachim 108a s.v. רב ששת. Sefer HaManhig Hilchos Pesach § 48 brings from Yerushalmi Pesachim 10:1 that some say Rabbi Yehudah HaNassi didn’t eat on erev Pesach because he was a firstborn
 Tur and Sefer HaManhig loc. cit.; Magen Avos Inyan 23 by the Meiri
 It’s further problematic because the custom is to refrain from fasting in Nissan (Soferim loc. cit.)
 The Be’er Yosef only suggested this לולי דברי הטור והפוסקים, only if the other authorities hadn’t given their approach
 Rashi to Deuteronomy 26:12, quoting Sifrei Devarim § 302
 Such as in Megillah 2:5 and Sotah 32b
 Tosafos Yom Tov to Ma’aser Sheni 5:10, quoting Seforno to v. 13
 This is the Be’er Yosef’s addition to what the Seforno explained
 See Pesachim 5:5
 Numbers 3:13
 Rashi ad. loc. v. 12, and Zevachim 112b. For more on the sin of the Golden Calf, see Exodus Chapter 32
 Yerushalmi Megillah 1:11 and Bamidbar Rabbah 4:8, as pointed out by the Be’er Yosef’s son
 See the rest of the Be’er Yosef, where he goes on to try to explain the more commonly known reason for the fast. He suggests that the firstborn in Egypt fasted as a form of prayer that they be spared from the plague of the firstborn. Therefore, we fast in commemoration of that, quite similar to Ta’anis Esther. See also the additional comments of the Be’er Yosef’s son. He notes that this latter approach also appears in the (then newly printed) Chiddushei Chasam Sofer to Pesachim 108a s.v. כל מעלי יומא דפסחא. He also cites many sources to support the Be’er Yosef’s first approach. He brings the various opinions about which kind of firstborn has the custom to fast (male or female, from father or mother, etc.), and how that connects to which firstborn served in the Temple. See there