Fast of the Firstborn (Hebrew: תַעֲנִית בְּכוֹרוֹת, romanized: taʿăniṯ bəḵoroṯ or תַעֲנִית בְּכוֹרים taʿăniṯ bəḵorim) is a unique fast day in Judaism, which usually falls on the day before Passover (i.e., 14 Nisan, a month in the Hebrew calendar; Passover begins on 15 Nisan). In modern times, the fast is usually broken at a siyum celebration (typically made after Shaharit), which, according to the prevailing custom, creates an atmosphere of rejoicing that overrides the requirement to continue the fast. Unlike all other Jewish fast days, only firstborn children must fast on this day.
This fast commemorates the salvation of the Israelite firstborns during the Plague of the Firstborn (according to the Book of Exodus, the tenth of the ten plagues wrought upon Biblical Egypt before the Exodus), when, according to Exodus (12:29): "In the middle of the night יהוה struck down all the [male] first-born in the land of Egypt, from the first-born of Pharaoh who sat on the throne to the first-born of the captive who was in the dungeon, and all the first-born of the cattle. "