(R. Elazar): Ben Beseira says that a Pesach slaughtered in the morning of Erev Pesach is Kosher, because the entire day is its proper time.

Pesachim 5a (Rava): We learn that Chametz is Asur on Erev Pesach from "Lo Sishchat Al Chametz Dam Zivchi" - do not slaughter Pesach while you still have Chametz.

Surely, we do not say that each individual must get rid of his Chametz before he slaughters. Rather, there is a fixed time for everyone.

Rav Sheshes used to fast every Erev Pesach.

Suggestion: He holds that the Mishnah forbids eating shortly before Minchah Gedolah, lest he neglect to offer Pesach, and he holds like Ben Beseira, who is Machshir a Pesach slaughtered in the morning.

Rejection: No. Rav Sheshes fasted because his nature was such that if he would eat (even earlier) during the day, he would not have appetite at night.

Maseches Sofrim (21:3): We do not fast in Nisan, just Bechoros on Erev Pesach, and Tzenu'im, to enable them to eat Matzah with appetite.

Milchamos Hash-m and Ran (Pesachim 15b and 16a): The correct text in the Yerushalmi says that R. Levi forbids eating Matzah on Erev Pesach. R. Yehudah (ben Beseira) forbids Chametz and Matzah,for he holds that the entire day is Kosher for Shechitas Pesach. R. Simon says that Rebbi did not eat Matzah on Erev Pesach due to R. Levi, and not Chametz due to R. Yehudah. We challenged this, for only R. Yehudah's Talmidim are concerned for his opinion. Rebbi was not his Talmid. Surely he holds like Chachamim! Rather, Rebbi was a Bechor. R. Yonah says, my father was a Bechor, and he did not eat on Erev Pesach. R. Tanchuma disagrees. He says that Rebbi was an Istandis (had a delicate nature). If he would eat during the day, he would not eat at night. He refrained on Erev Pesach, in order to eat Matzah at night with appetite.

Rosh (Pesachim 10:19): The Yerushalmi suggested that Rebbi was a Bechor, but concluded that he was an Istandis. This supports the custom for Bechoros to fast on Erev Pesach, like it says in Maseches Sofrim.

Tosfos (Pesachim 108a DH Neima): Even though there is no Pesach nowadays (in the days of Rav Sheshes), the Isur that applied at the time of the Mikdash still applies. Even though R. Yehudah is Machshir only b'Di'eved, but l'Chatchilah one must slaughter after midday, due to "Lo Sishchat Al Chametz", and we do not find anyone who forbids Chametz before midday, he holds that we must be stringent not to eat Chametz from the morning.

Terumas ha'Deshen (126): The Roke'ach says that when Erev Pesach is on Shabbos, Bechoros fast on Thursday. A Chacham wanted to prove that they do not fast at all. Amora'im in the Yerushalmi argued about why Rebbi fasted. If Bechoros must fast (before Shabbos) even when Erev Pesach is on Shabbos, they should have known based on whether he fasted beforehand! Fasting before Shabbos would not help to eat Matzah with appetite on Motza'ei Shabbos. It would be only for Ta'anis Bechoros. This is not a proof. Perhaps the Amora'im did not know what he did when Erev Pesach was on Shabbos! We find that Benei Beseira did not know whether Korban Pesach overrides Shabbos when Erev Pesach is on Shabbos. We must say that for many years, Erev Pesach was not on Shabbos, and they forgot, even though it was a public matter. All the more so we can say that a private matter (Rebbi's fasts) was forgotten, for also in his days, for many years Erev Pesach was not on Shabbos!

Shulchan Aruch (OC 470:1): Bechoros from the father or mother fast on Erev Pesach.

Beis Yosef (DH Garsinan): The Mordechai permits a Bechor to eat Targima (Mezonos, meat and fish, or Peros). This is not the custom.

Magen Avraham: A baby born after a Nefel (stillborn baby) must fast, for he is a Bechor for inheritance. Olas Tamid (1) permits Bechoros to eat at a Seudas Bris Milah. One who is stringent will be blessed. In our lands, the custom is to be stringent.

Shulchan Aruch (ibid): Some say that even a female Bechor fasts.

Rema: This is not the custom.

Beis Yosef (DH v'Chasav): Avi ha'Ezri and Hagahos Maimoniyos say that also a paternal Bechor fasts. Maharik says that a female Bechorah fasts. The Agudah brings a proof from Bas Paro.

Gra (1): A paternal Bechor is Kodesh for inheritance. A maternal Bechor has Kedushah regarding Pidyon ha'Ben. The Medrash says that if a Mitzris had children from 10 single men, or if a Mitzri had children from 10 women, all of them died. R. Avin says that Bas Paro was a Bechorah,and she was saved through Moshe's Tefilah. The custom is that women do not fast, for the Torah did not give Kedushah to female Bechoros.

Mishnah Berurah (1): The fast commemorates the miracle of being saved from Makos Bechoros.

Mishnah Berurah (2): Even though the oldest of the house died in Makos Bechoros, Chachamim did not obligate him to fast. One born after a Vadai Nefel must fast, but not one born after a Safek Nefel (it had a full term pregnancy, but died within 30 days). A Bechor Kohen or Levi must fast, even if he is only a maternal Bechor, even though he is not a Bechor for inheritance or redemption, still he is called a Bechor. Maharil says that one (who did not fast) must compensate afterwards, for it is a Ta'anis Tzibur. If one's head or eyes hurt, he need not fast. If the fast is difficult for someone, and afterwards he can eat only light things and a small amount, and it is likely that he will not properly fulfill eating Matzah and Maror and the four cups, it is better that he not fast, so he can fulfill the Mitzvos of the night properly. It is best that he eat only Targima. Bechoros who fast say Aneinu in Minchah.

Kaf ha'Chayim (4): If the oldest of the house wants to fast, this is a (proper) custom.

Kaf ha'Chayim (3): It is a mere Safek if one born through Caesarean section must fast, so he may be lenient.

Kaf ha'Chayim (5): If we do not know which of two twins was born first, this is a Safek mid'Rabanan so we are lenient.

Kaf ha'Chayim (8): They must complete the fast until night. Others say that one should not be stringent about matter without a source in the Bavli, especially if it could hinder the Torah Mitzvah to eat Matzah.

Shulchan Aruch (2): If Erev Pesach is on Shabbos, some say that Bechoros fast on Thursday, and some say that they do not fast at all.

Beis Yosef (DH v'Chasuv): The Terumas ha'Deshen and Roke'ach say that they fast, and ahe Agur citing Mahari Molin, but he says that his father did not fast, since it was pushed off and it is only a custom.

Gra: We say in Megilah (4b) that since it (Kri'as ha'Megilah) was pushed off (from Shabbos), it is ii. pushed off (back to Thursday). Parents fast for the Bechor, because the Bechoros died due to the sins of their parents - "Hinei Anochi Horeg Es Bechorecha."

Magen Avraham (1): If the fast is difficult, they may eat before checking for Chametz at night, or they can make a Shali'ach to check.

Rema: One should follow the first opinion. The custom is that if the father is a Bechor, the mother fasts for her firstborn son while he is a minor. If the father is not a Bechor, he fasts for his Bechor until his Bechor matures.

Mishnah Berurah (9): Some say that the mother need not fast, for the father's fast counts for himself and also for his son. One may be lenient if it is painful, and all the more so if she is pregnant or nursing and the fast pains her. She may be lenient even if she has no husband to fast for her son. One who gave birth in the last 30 days should not fast in any case. However, if she fasted once this is like a vow, and she must permit it.

Mishnah Berurah (10): One need not fast for a Bechor less than 30 days old. Bechoros may eat at a Seudas Mitzvah where this is the custom. Where they are stringent, one who wants to eat at a Seudas Pidyon ha'Ben or Bris Milah needs a Heter (an important custom is like a vow). The Mohel, Sandek and baby's father need no Heter, for it is Yom Tov for them, but they must compensate after Pesach. In many places Bechoros are lenient to eat even at a Seudas Siyum, even if they did not learn it, since it is a Seudas Mitzvah for the one who learned.

Kaf ha'Chayim (10): A Bechor who understands what is said at a Siyum may eat there.

Kaf ha'Chayim (23): One who fasts for his son may be lenient when Erev Pesach is on Shabbos.

Yabi'a Omer (OC 1:25:1): In our text of the Yerushalmi, R. Yonah says that his father was a Bechor, and he ate on Erev Pesach. Based on this, Bechoros need not fast on Erev Pesach. However, in the text of the Ramban and Ran, R. Yonah holds that they must fast. The Bavli says that Rav Sheshes did not eat due to his nature. It did not say so about Rebbi. This connotes that it holds that Rebbi did not eat because he was a Bechor.

Note: Perhaps the Bavli was unaware that Rebbi did not eat on Erev Pesach! The Gemara retracted about why Rav Sheshes did not eat. It never suggested that he fasted because he was a Bechor!