In commemoration of the slaying of the Egyptian firstborn before the Exodus, our Sages require the firstborn sons of our people to fast on the day preceding Pesach, from sunrise to nightfall. Similarly, when a firstborn son is below the age of 13, his father is required to fast on his behalf. If Pesach begins Saturday night, the Fast of the Firstborn is held on the preceding Thursday.

A fast of this nature may be broken to participate in a seudas mitzvah, a feast held in celebration of a mitzvah, e.g., a bris (circumcision) or the conclusion of a tractate of the Talmud. Once a person participates in such a feast, he may continue eating for the entire day. Our Rabbis have recommended that the firstborn and the fathers of the firstborn take this option rather than actually fast. In most communities, such a feast is organized after the morning service on the appropriate day.