Written by Rabbis Eli Gersten, Yaakov Luban, and Moshe Zywica of the Orthodox Union
It is an ancient and widespread custom for the firstborn (bechor) to fast on the day before Passover. This commemorates the miracle which spared the firstborn Jewish sons from the plague that struck down the firstborn sons of the Egyptians. Usually, the fast is broken at a siyum, a meal celebrating the completion of a tractate of the Talmud or other learning. Unlike all other Jewish fast days, only firstborn males are required to fast on the Fast of the Firstborn.
In an ordinary calendar year, the firstborn is required to fast on Erev Pesach. This year’s calendar, with Shabbat being Erev Pesach, is different. The custom is to fast in advance on Thursday (Rema O.C. 470:2).
Why not Friday, which is closer to the actual Erev Pesach? To avoid starting Shabbat while fasting (Aruch HaShulchan 470:4).
Bedikat chametz (the search for the chametz) is performed Thursday evening. If it is too difficult to fast until after bedikat chametz, one may snack before beginning the bedikah (M.B. 470:6).
As in other years, the firstborn may participate in a siyum (a completion of a tractate of the Talmud) on Thursday, which would exempt him from fasting (M.B. 470:10).