Siman 470

It is stated in Masekhet Soferim (chapter 21) that one does not fast in Nisan, except for the firstborns on Erev Pesach, and the modest ones on account of the matzah so that they may enter into it with desire, see there. The firstborns [fast] in memory of the plague of the firstborns; and as for the modest ones, it appears that the intention is for those of delicate constitution, for when they eat during the day they do not eat at night with appetite, as it is stated in the Gemara (108a): Rav Sheshet would sit in a fast every Erev Pesach, because he was of delicate constitution. For if he tasted anything in the morning – the food in the evening would not benefit him. And in the Yerushalmi at the beginning of "Arvei Pesachim" it is stated: Rabbi would not eat on Erev Pesach... because he was a firstborn. Rabbi Mana said to Rabbi Yona: My father was a firstborn and he would eat... rather [it is] from this: Rabbi was of delicate constitution, when he ate in the day – he would not eat in the evening. And why would he not eat? So that he would enter into the matzah with desire. Thus far his words. And even though if it is because of a delicate constitution – he does not require a fast, the Tosafot wrote there and this is their wording: Rav Sheshet would sit in a fast – not literally, but rather he guarded himself from eating. Alternatively: he would actually accept a fast upon himself, so that he would be reminded and careful not to eat. Thus far their words. (And it requires further study regarding the Rosh, who wrote that there is support from the Yerushalmi. For behold, the conclusion is not so.)

And based on the words of Masekhet Soferim, the Tosafot, the Tur, and the Shulchan Aruch wrote that the firstborns fast on the eve of Pesach, whether a firstborn from the father or a firstborn from the mother. For anyone upon whom the name "firstborn" applies, whether a firstborn for inheritance such as from the father or a firstborn for the kohen such as a firstborn to the mother—he fasts. For in Egypt all types of firstborns were killed, whether from the father or from the mother, as Rashi explained on the verse "for there was no house where there was not someone dead." And even the head of the household is called "firstborn," see there in Rashi. And in Psalms (78) it is written: "And He smote every firstborn in Egypt, the first of their strength in the tents of Ham." And "the first of his strength" is a firstborn from the father, as it is written: "for he is the first of his strength, the right of the firstborn is his." And Yaakov said regarding Reuven: "my might and the first of my strength." And despite all this, the Holy One, Blessed be He, did not command to sanctify firstborns except for a firstborn from the mother, because he is "the first to open the womb." And it seems to me the reason is because the Holy One, Blessed be He, sanctified "from the firstborn of man unto the firstborn of beast," and regarding a beast only "the first to open the womb" is relevant, as is understood. Therefore, regarding man as well, only the first to open the womb was sanctified.

And there is one who says that even a female firstborn fasts, for certainly in Egypt they too were killed in the plague of the firstborns. And the proof is that the Midrash says that Bithiah the daughter of Pharaoh was saved in the merit of Moses; it follows that the plague of the firstborns applied to females as well. Nevertheless, the custom is not so, for the reason that regarding females, the Holy One, Blessed be He, did not command to sanctify them, only the males. And one who comes after a miscarriage must fast, since he is a firstborn for inheritance. And so too firstborns who are Kohanim and Leviyim—they must fast, even though they do not require a redemption of five sela’im, for they are no less than a firstborn from a father. And if there are ten firstborns, there is one who says that the shaliach tzibbur says "Anenu." But it is not proper to do so, for if so, it would be like a public fast, and this is forbidden in Nisan. Rather, the individual says "Anenu" in Shomea Tefillah. And they do not read "Vayechal."

And if Erev Pesach falls on Shabbat, the bechorim are accustomed to fast on the Thursday prior. And some say that since it was pushed off, it is pushed off, for it is only a custom. However, the custom is to fast according to the first opinion. And they do not fast on Erev Shabbat, so that one does not enter Shabbat while suffering. And when they fast on Thursday, and one must perform bedikat chametz and the fast is difficult for him—he may taste something before the bedika (Magen Avraham). Our teacher, the Rema, wrote that the custom is that when the father is a bechor—the mother fasts on behalf of her minor son when he is a bechor. And if the father is not a bechor—the father fasts for his son until he grows up. Thus far his words. And currently, this custom does not exist.

Regarding a brit milah or pidyon haben—the firstborn may eat. And there is one who wrote that specifically those related to the mitzvah, such as the father of the child, the sandak, the mohel, and those similar to them (Magen Avraham). And there is one who says that one who is lenient for any person regarding a seudat mitzvah has lost nothing (Pri Chadash). However, now for several generations they have been completely lenient regarding the fast of the firstborns, and that is with a siyum of a masekhet. And not only the one who learns it, but the firstborns gather around the one completing it, and they complete it with him and eat. And this has spread in all countries. And it is not known from where they have the basis to be so lenient, if we do not say that it is due to the weakness of the generation, and the burden is great on Erev Pesach, and the eating of the maror is also not good for health. Therefore, they consider themselves as those who are unable to fast. And since in the Gemara this is not mentioned at all, and also in the Yerushalmi the conclusion is that they do not need to fast, and it is only a custom based on Masekhet Soferim—therefore the sages of the generation did not protest this. And it requires further study.