Fast of the Firstborn, The sunrise to sunset Ta'anit B'khorot (Hebrew: תענית בכורות), Fast of the Firstborn, is a unique fast day in Judaism which usually falls on the day before Passover (i.e. the fourteenth day of Nisan, a month in the Jewish calendar. Passover always begins on the fifteenth of the Hebrew month). This is the only fast that applies to just a segment of the community which are all males who are the firstborn children in their families. The father or mother of a child too young to fast, fasts for him (note: since it is forbidden to abstain from eating on Shabbat (except for Yom Kippur), when Erev Pesah falls on Saturday night, the fast takes place on Thursday).

Usually, the fast is broken at a Siyyum celebration (typically made at the conclusion of the morning services), which, according to prevailing custom, creates an atmosphere of rejoicing that overrides the requirement to continue the fast (see Breaking the fast below). Unlike all other Jewish fast days, only firstborns are required to fast on the Fast of the Firstborn.

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 Ancient Egypt prior to the Exodus of the Children of Israel), when, according to Exodus (12:29): "...God struck every firstborn in the Land of Mitzrayim (Ancient Egypt)...."[3]

https://en.wikipedia.org/wiki/Fast_of_the_Firstborn