The Talmud Yerushalmi records two reasons for fasting on Ereb Pesah. First, one Rabbi fasted because he was a Bechor (firstborn) and did so to commemorate the miracle that Hashem spared the Jewish first born during the final plague. The Gemara there also brings a case in which a Rabbi fasted on Ereb Pesah, because he was an "Istinis" (finicky). If he would eat during the day, he would not have any appetite to eat Masa on the Seder night. The Gemara concluded that this is the actual basis for the fast. However, Masechet Sofrim does record a fast on Ereb Pesah connected with the firstborn, although it is not mentioned in the Talmud Bavli.
Shulhan Aruch (470) brings down this custom for both paternal and maternal firstborn men to fast. The fast begins at dawn until Kiddush on the Seder night. Women are exempt from fasting. If a non-firstborn father has a child who cannot fast, he should fast on his behalf. If the father is also a firstborn, the mother should fast for that child. Shulhan Aruch there also rules that one who will not have an appetite on the Seder night, if he eats during the day, should also fast.
The Shulhan Aruch, following the Rishonim, does not cite any dispensations or exemptions for this fast. The later authorities saw that the generations were weaker and would have trouble conducting the Seder on an empty stomach. Therefore, they found Halachic "loopholes" to avoid fasting, although the Aruch Hashulhan did not look favorably at such practices.
The most reliable way for a firstborn to become exempt is to attend a Siyum Masechet (conclusion of studying a tractate), in which the volume was completed naturally coinciding with Ereb Pesah, according to the regular schedule of the learner. All who participate in such an event, even more than a Minyan, can break their fast at the Siyum. Hacham Bension writes that they should each have a Kezayit of Mezonot, whereas Hacham Ovadia is lenient to even have a Kezayit of fruit. All agree that drinking a Revi'it of wine constitutes participation in the celebration. Hacham Bension conceded that a Kezayit of dates is also valid, since they satiate. Even if one came late to the Siyum, Hacham Ovadia rules that he is still included, as long as words of Torah are still being said.
Hacham Ovadia rules that even finishing a Masechet of Mishna with the commentary of the Bartenura and some of the Tosafot Yom Tob is considered a bona fide Siyum for the learner to break his own fast, but it is not sufficient cause for celebration that others can break their fast as well. He also rules that finishing a volume of the Zohar HaKadosh is a valid Siyum, even if he did not understand what he read. Rav Moshe Feinstein (Russia – New York, 1895-1986) ruled that finishing a book of the Nevi'im (Prophets) is a bona fide Siyum, although Hacham Ovadia did not explicitly mention that this can count.
Hacham Ovadia is also lenient regarding breaking the fast by attending a Seudat Misva, such as a Brit, Pidyon Haben, Bar Misva or Sheva Berachot.
It should be noted that Hacham Bension is a little stricter in these matters. He rules that if one gauges himself that he can fast and still function, then he should do so.
There is an unwritten custom in our communities for the men to bring a piece of cake home to the women of their household. They call it "the magic cake." Hacham Bension explains that this custom is a way of also including firstborn women in the Siyum, even though they are not actually obligated to fast.
When Pesah begins on Friday night, the fast is on Friday, even though fasts are generally not set for Fridays. However, if Pesah begins on Mosa'eh Shabbat, the fast is moved up to Thursday.
