What is the logic behind the first born fasting on erev Pesach? Is it a form of celebration? If so, would not a feast be better? Is it a form of teshuva? If so, for what sin? Is it a form of thanks? If so, by what logic is a fast a form of thanks? Also, why is the fast limited to the first born? None of us alive today were personally passed over.
I spent a while researching this along with some similar questions. Here's what I came up with:
The vast majority of Rishonim (A notable exception is the Rambam who doesn’t quote this Halacha), including the Tur and Shulchan Aruch (O.C. 470:1), Pasken based on Meseches Sofrim (21:3) [That is the primary source. However, some Rishonim such as the Mordechai (Pesachim 610) and the Orchos Chaim (Chelek Aleph Din Arvei Pesachim Veshaar Yamim Tovim os 13) quote Yerushalmi Pesachim 10:1 as the source. A discussion as to whether or not this is a valid source is beyond the scope of this discussion. See Rosh Pesachim 10:19 with Kobran Nesanel (ad loc. os Samech and os Ayin), Birkei Yosef (Orach Chaim 470:1), and Sefer Taanis Bechorim (pg. 4-11).] that firstborns should fast on Erev Pesach.
The Tur (ibid.) explains that the reason for the fast is to commemorate the fact that the Jewish firstborns were saved by makkas bechoros. This is a strange thing to need to commemorate. The makkos simply did not apply to us. We did not need to commemorate that the Jews were able to drink water while all the Egyptians drank blood by makkas dam, that the wild animals didn’t enter Goshen by makkas arov (Shemos 8:18), that not one animal belonging to a Jew dies by makkas dever (Shemos 9:6), that the hail didn’t enter Goshen by makkas barad (Shemos 9:26), or that all of the Jews had light by makkas choshech (Shemos 10:23). Why should the firstborn Jews have died? The Jews weren’t affected by any of the other plagues! (Many Jews died by makkas choshech, but they weren’t affected by the plague of darkness itself. Hashem merely used the cover of darkness to remove them without the Egyptians being aware that Jews were dying left and right.)
The Orchos Chaim (Chelek Aleph; Din Arvei Pesachim Veshaar Yamim Tovim; os 13) writes that the reason why the firstborns were specifically targeted by makkas bechoros is because they were the priests for the avodah zarah there. Based on this, the Orchos Chaim points out that it is likely that among the Jews, many of which were idol-worshippers as well, many firstborns would have been priests for avodah zarah (See Yechezkel 20:7 [as explained by many commentators] and Shemos Rabbah 21:7). If so, since this was primarily an attack on those that were the priests for avodah zarah, the Jewish firstborns should have been killed were it not for Hashem’s mercy.
However, the explanation that Taanis Bechorim commemorates the salvation of the firstborns remains troubling. We don’t generally commemorate being saved by fasting. Generally, when we are saved, we commemorate that by eating. As Alan King said, “A summary of every Jewish holiday: They tried to kill us, we won, let’s eat!” Shouldn’t firstborns be celebrating their salvation through feasting, as opposed to mourning it through fasting?
To make matters worse, the Midrash Rabbah writes (Shemos Rabbah 19:7) that we do not mourn the suffering we had in Egypt. This is comparable to a king who married a woman from across the sea. On her way to the king her boat was rocked by terrible waves for days making her fear for her life, but in the end she survived. When she finally reached the king and informed him what had happened to her on the way the king told her “I don’t want you to commemorate all the suffering you experienced getting here. Instead, I want you to remember the day that you were saved and make it a day of celebration every year.” This is understandable. Nobody wants to start off their marriage with the wife focusing on how miserable her journey to the wedding was. That would make the whole marriage start off on the wrong foot. Hashem, as well, did not institute any days of mourning to commemorate the terrible suffering we experienced in Egypt. [We do eat marror and recount the bad times we had (maschil begenus umesayem beshevach), but the goal is not to mourn the suffering we endured under the hands of the Egyptians but rather the goal is to help us appreciate our miraculous salvation.] We only celebrate the salvation. If so, why do we fast to commemorate what the bechorim went through?
Based on the strength of these questions, some, such as the Sefer Pe’er Yitzchak (25:4), have suggested that the entire concept of Taanis Bechorim is based on a ta’os sofer (The letter Gimmel from the word "מתענגים" got left out and turned into "מתענים") and indeed, the author brings a number Gedolim who were firstborns did not bother fasting or even attending a siyum or other seudas mitzvah because they too felt it was a ta’os sofer. However, being that virtually all major Halachic works over the last thousand years have felt that Taanis Bechorim is halacha lemaisa it behooves us to understand what the purpose of it could possibly be.
Shu”t Lecha Shlomo suggests that generally we don’t fast to commemorate various tribulations we were saved from because we ourselves did mitzvos or teshuva and merited to be saved. However, by yetzias Mitzraim we were utterly bereft of Mitzvos and it was only because Hashem graciously gave us the Mitzvos of dam Pesach (the blood of which was placed on the lintel and doorposts) and dam Milah that we were able to be saved. Therefore, Chazal instituted that firstborns should fast in order to diminish their fat and blood as a reminder that their blood should have been spilled.
Perhaps we can suggest in a similar vein that we don’t generally need to mourn the terrible things that happened to us that we were saved from because the purpose of all the fast days that we mourn nowadays is as the Rambam (Hilchos Taanis 5:1) writes “יש שם ימים שכל ישראל מתענים בהם מפני הצרות שארעו בהן כדי לעורר הלבבות לפתח דרכי התשובה ויהיה זה זכרון למעשינו הרעים ומעשה אבותינו שהיה כמעשינו עתה עד שגרם להם ולנו אותן הצרות. שבזכרון דברים אלו נשוב להיטיב שנאמר (ויקרא כו, מ) "והתודו את עונם ואת עון אבתם" וגו'.” The purpose of a fast day is to remember the ways our ancestors suffered for their misdeeds which they did not correct and we ourselves are following in their footsteps and sinning.
If our ancestors had dealt with the problem head on by doing Teshuva, there would be no need to mourn this day. However, by makkas bechoros our ancestors never dealt with the problem by doing teshuva. As the Orchos Chaim said, the firstborns deserved to die for worshipping avodah zarah, and even when we came to the sea before krias Yam Suf the sea was not going to split for us because we were idolaters (Shemos Rabbah 21:7). Indeed, even after Matan Torah the firstborns still struggled with avodah zarah and were the first ones who sinned by the Golden Calf (Yerushalmi Megillah 1:11). Thus, since the entire salvation happened without the firstborns ever fully repenting for their sins it remains a day of mourning, although since the firstborns were not actually killed out of Hashem’s mercy, we don’t remember the tragedy but rather that the firstborns were miraculously saved from receiving the fate they deserved and therefore Taanis Bechorim is not treated with the same stringency as other fast days.
There are also several other reasons for Taanis Bechorim that circumvent the entire issue of why firstborns fast to commemorate the miracle of their salvation altogether by providing other reasons why firstborns fast:
The Chassam Sofer (Pesachim 108a d”h “kol maali yoma depischa”) suggests that Taanis Bechorim is similar to Taanis Esther. The same way that on Taanis Esther the Jews fasted as a merit that they should not be killed in battle so too the firstborns fasted on Erev Pesach as a merit that they should not be killed for their sins, and nowadays we do the same to commemorate that. The Chassam Sofer adds that it is likely that fathers fasted on behalf of their firstborn sons who were ketanim to ensure that they wouldn’t die for their father’s sins which is why nowadays as well fathers fast on behalf of their firstborns under bar mitzvah.
In the journal Otzar Hachaim (5691 pages 96-99) another reason is suggested by Rav Ehrenreich. At the point that Hashem slaughtered all the firstborns in Egypt He sanctified all of the Jewish firstborns (Shemos 13:2, Bamidbar 3:13 and 8:17) and made them the Kohanim (Shemos 19:22 and 24:5 with Rashi). However, when they sinned by the chet ha’egel the bechorim lost their right to be the priests and that right was given to Shevet Levi (Bamidbar Rabbah 3:3, 3:10, and 4:8). Therefore, the bechorim fast to demonstrate their sorrow over the loss of their incredible opportunity to serve Hashem as Kohanim. It is also possible that this is what the Tur meant when he said that the fast is to commemorate the fact that the bechorim were saved. They are remembering the fact that they were saved by Hashem by way of Hashem sanctifying them which they subsequently lost.
This explanation also explains why, unlike all the other fasts, if one attends a siyum they no longer need to fast. Even though the firstborns are no longer capable of bringing the korbanos, learning Torah is equivalent or perhaps even greater than bringing korbanos (Sifri Devarim 41, Menachos 110a, Avos D’Rebbi Nassan 4:1). Through making or even attending a Siyum the bechorim are showing that despite having lost their incredible opportunity to serve Hashem through bringing korbanos they can still serve Hashem on at least the same level through learning Hashem’s Torah.
Rav Shlomo Zalman Auerbach (Pa’amei Kol Torah, cited in Sefer Taanis Bechorim page 22) points out that claiming that the reason for Taanis Bechorim is to remember that the firstborns were saved by makkas bechoros is difficult because makkas bechoros did not happen on the 14th of Nissan, it happened on the 15th. The reason we commemorate it on the 14th cannot merely be because the 15th is Yom Tov so they moved it to the 14th. If that were the case the fast should have been moved to the 13th since the 14th of Nissan is also somewhat of a Yom Tov, as it is Erev Yom Tov and the day that the Korban Pesach was brought. There are even restrictions against doing various forms of melachah on the 14th.
Furthermore, the same way we see that when Purim is on Sunday, Taanis Esther is moved back to Thursday, as opposed to Friday (Shulchan Aruch, Orach Chaim, 686:2) and when Erev Pesach falls out on Shabbos, Taanis Bechorim is shifted back to Thursday as well (470:2). The Aruch Hashulchan (470:4) Mishna Berura (470:6) explain that the reason Taanis Bechorim is shifted back to Thursday instead of Friday is because since its anyway not in the proper time it is preferable to distance it from Shabbos so that the bechorim do not enter into Shabbos while fasting. Based on that, if Erev Pesach were not the real time for the fast it would seem inappropriate to reschedule the fast to Erev Pesach thereby causing the bechorim to enter into Pesach while fasting.
Furthermore, if Taanis Bechorim was really because the bechorim were saved it should be the descendants of those bechorim who fast, not other bechorim who were descended from people who weren’t bechorim at the time of makkas bechoros. Future bechorim wouldn’t have been affected by the fact that the bechorim were spared unless their ancestors were bechorim at the time of makkas bechoros, so why are they the ones who fast?
Rav Shlomo Zalman therefore suggests that the reason for Taanis Bechorim is actually because when the bechorim came along with the rest of Klal Yisrael to bring their korban Pesach on Erev Pesach they were incredibly pained, depressed, and embarrassed about the fact that they lost their opportunity to be ones performing the avodah in the Beis Hamikdash and instead had to merely watch as the Kohanim did the avodah in their place. This was especially difficult for them because the korban Pesach that they were bringing actually celebrates the very moment that they became sanctified. Because of the bechorim’s incredible pain and sadness they did not eat or drink at all that day, and they instituted it as a day of mourning for all those who would have been the ones doing avoda as well.
(The reasons of Rav Shlomo Zalman and Rav Ehrenreich both do not explain why firstborn Kohanim and Leviyim fast. They are still doing avodah in the Beis Hamikdash. Furthermore, these explanations do not explain why boys who are firstborns only from their father fast, as they were not sanctified by makkas bechoros (this is why they do not need pidyon haben). It is likely that these reasons are not intended to be the only reasons but rather complimentary reasons. See Sefer Taanis Bechorim (5:1) for more on why it is not just the bechorim from the mother that fast.)
Perhaps we can weave elements from several of these reasons together to form another beautiful explanation for Taanis Bechorim. Chazal tell us (Rosh Hashana 11a) that the same way we were redeemed in Nissan we will be redeemed again in Nissan at the end of our current exile. [See Drashos HaRan (end of Drush 3) that we pasken that we will be redeemed in Nissan like Rebbi Yehoshua says and not in Tishrei as Rebbi Eliezer says.] Yalkut Shimoni (Shemos 12:42 and Yeshaya Remez 436) and Rashi (Eruchin 11b d”h “zechus”) imply that this means that we will be redeemed on the exact same dates as our original redemption from Egypt. When we are redeemed and the Beis Hamikdash is rebuilt the service in the Beis Hamikdash will be restored to the firstborns (Ohr Hachaim [Bereishis 49:28 d”h “asher diber” and Bamidbar 3:45 d”h “vihayu li”], Tikkunim Chadashim of the Ramchal , and Ahavas Yehonasan [Haftaras Emor]).
However, although the redemption culminates on Seder night, it doesn’t start then. Throughout Erev Pesach we begin experiencing the redemption of Pesach. Chametz starts to become forbidden to us, we become forbidden to do serious melachah that is not for Pesach purposes (Shulchan Aruch, Orach Chaim 468:1-3), and this is the time that we would bring the korban Pesach should the Beis Hamikdash be rebuilt. Furthermore, in Egypt the effects of makkas bechoros started before Pesach with the firstborns waging war against their fathers (Midrash Aggada Shemos 11:5, Midrash Tanchumah [Buber] Bo 18, Midrash Tehillim 136, Yalkut Shimoni Tehillim 136, Pesikta Rabsi 17). (See also Zohar Pinchas 249a that Hashem’s throne will become complete on Erev Pesach with the wiping out of Amalek.) Thus, this is a very opportune moment for the firstborns to remember the way they had been entirely spared in Egypt many years earlier by way of being taken into the service of Hashem and they fast that day in the zechus that the Beis Hamikdash should be rebuilt and they should be worthy to be the ones doing the avodah and bringing the korban Pesach on this day in the Beis Hamikdash just as they had been many years earlier.
[See Sefer Taanis Bechorim (page 23) for more reasons for Taanis Bechorim.]
Wikipedia, which also lists sources, states:
The origins of the Fast of the Firstborn are found in the Talmud, and the custom may have existed even prior to Talmudic times. The primary Talmudic source quoted for this custom is found in Tractate Soferim (21:3), where it is stated that firstborns fast "in commemoration of the miracle that they were saved from the Plague of the Firstborn." Rabbeinu Asher, in his comprehensive halakhic commentary on the Babylonian Talmud (Pesachim 10:19), as well as Rabbeinu Aharon HaKohein in his Orchot Chayyim (p. 76, §13), quote the Jerusalem Talmud (Pesachim 68a) as an additional source for the fast.
Rabbi Yehuda Grunwald (Rabbi of Satmar and student of the Ketav Sofer) suggests that the firstborn Israelites fasted in trepidation in advance of the Plague of the Firstborn; despite a divine guarantee of safety, they felt a need to fast in repentance to achieve greater divine protection. Rabbi Grunwald thus posits that this was the precedent for the Fast of the Firstborn (Zichron Yehuda, vol. 1. §133).
The "Halocho a Day" website last week had this reason:
Halocho #1550 - Why do the firstborns fast?
Why don't firstborns celebrate the fact that they were saved from the 10th plague - the smiting of Egyptian firstborns?
On Erev Pessach - Friday next week - all firstborns will fast in memory of them fasting in Egypt on Erev Pessach, to ensure they wouldn't be punished along with the Egyptians in the 10th plague.
I do not know a source for first borns fasting in Egypt.