Yerushalmi Pesahim 10:1 records that Rabbi used to fast on the day before Pesah because he was a first born. By contrast, R. Yonah, despite being a first born is said to have disregarded this practice. The practice is presented as normative and common in Massekhet Soferim 21:1.
Regarding the scope of this practice, the discussion centers around how precisely we track the practice with the midrashic memory of who died in the plague of the first born in Egypt. The Torah says—כי אין בית אשר אין שם מת, that there was no house without someone who died. This suggests a very expansive definition of who died in the plague, since it was certainly not the case that every house had a first-born son born to its father, which would be the standard definition of first born in the context of inheritance and the patriarchal societies of the day. Midrashic sources thus suggest a much broader scope to the plague. Shemot Rabbah 18:3 describes the plague of the first born as affecting all first born, both matrilineal and patrilineal. It goes further, suggesting that first born daughters were killed as well, and that only Bityah, Pharaoh’s daughter, was saved on account of Moshe. Peskita Rabbati cites R. Abba b. Hama as saying that a house lacking a first born would see its head of household (גדול הבית) struck down. [The Mekhilta has a different approach to the phrase כי אין בית אשר אין שם מת, suggesting a desecration of already dead first-born and their tombs resulted in a feeling of catastrophe and death in every Egyptian home.]
Note that these various categories are different in terms of their resonance with other legal categories. Patrilineal first-born are significant with respect to inheritance, the בכורה follows the first born of the father, irrespective of how many prior children a given wife has had with other men. Matrilineal first-born are significant with respect to פדיון הבן, the redemption of the first born son. In that context, only the mother’s first born is relevant, such that a father’s first child in a second marriage where the wife already had a child with another man is exempt. By contrast, female first-born never otherwise have a legal status, and the same goes for head of household (outside of some peripheral laws related to mourning and the start of shiva). This unevenness plays a role for some later interpreters in deciding which parts of these midrashim should be actualized in the context of the fast the day before Pesah. On some level, the question is: Should the set of people who fast be seen as derivative of other categories in halakhah, or should the aggadah and the social realities it assumes and describes drive the practice?
Ra’aviah II:525, based on these midrashim, says that the first born of either the father or the mother must fast, since the plague in Egypt was all-encompassing of both types of first born. However, he says that heads of household need not fast; we do not go that far in putting the midrash’s mythic memory into practice. Shulhan Arukh OH 470:1 follows this view.
The children of kohanim/b’not kohanim and levi’im/b’not levi’im are exempt from פדיון הבן. However, Responsa Maharil #14 says that these first born should fast—even if they are not the father’s first-born, such that they have no legal status of a first-born in any area of law—since at the time of the plague in Egypt, they had no special status vis-à-vis regular first born sons and therefore must fast to reflect gratitude and trepidation for having been saved from the plague.
Agudah Pesahim #91 follows the midrash in saying that first-born daughters must fast as well. Responsa Maharil #14 reports that his father-in-law in fact made his daughter, Maharil’s divorcee (!), fast on the day before Pesah. However, Sefer Maharil Erev Pesah #4 seems to suggest that most legal authorities did not require this. Shulhan Arukh OH 470:1 reports the view of the Agudah. Rema states that it is not the practice to require daughters to fast. Gra grounds this in the fact that we have no evidence of the Torah ever prescribing a special legal status to the female first-born in any other area.
Finally, the laws of first-born are more broadly affected by miscarriages and stillbirths. In general, any woman who has miscarried a significant way through her pregnancy or who has a stillbirth does not redeem the next son born to her. However, Magen Avraham states that a first-born son after a miscarriage still fasts on the day before Pesah, since he is still a first-born for purposes of inheritance (through his father, presuming he is indeed his father’s first born). He here appeals to other areas of law to fill in details with respect to this fast. Hok Ya’akov leaves unresolved the case of a Caesarian-section birth, since such a child neither has a פדיון הבן nor is considered the first-born for purposes of inheritance (an interesting discussion in its own right). Based on reinforcement from other legal categories, it would seem we should exempt, and yet based on the person’s social status and how they would have been regarded at the time of the plague of the first born in Egypt, they would certainly have been treated as a first born. Kaf Hahayim says we should resolve this doubt leniently and not require such a person to fast. Shevut Ya’akov II:16 clarifies that any live birth, even if the child dies within 30 days, is considered the first-born for purposes of eliminating the status of subsequent children from that category.
In many contemporary communities, this fast is deliberately evaded by attending a celebration completing learning, a tradition that goes back to at least the 16th century. In this context, we might think not about who is expected to fast, but onto whom we project expectations of first-born status and how we maximize our tangible connection to our memory of the plague of the first-born. We perhaps most fully feel the after effects of כי אין בית אשר אין שם מת by making sure that at least one person from every home, and especially all of our first born sons and daughters, begin the day before Pesah at minyan and joining in to an experience of meaningful learning. We can thus enter, one home at a time, into Pesah with a profound sense of gratitude for our redemption through God’s hands.