In the tenth and final plague inflicted upon Egypt, G-d killed the firstborn
in all of Egypt. But, as in all the plagues brought upon Egypt, the Children of
Israel were spared. In the Plague of the Firstborn, not one Jewish firstborn
died. To express their gratitude, all firstborn males fast on the day before
Passover (Erev Pesach). The fathers of firstborn boys under the age of 13 fast
in their stead.
The prevailing custom, however, is for the firstborn to exempt themselves from the obligation to fast by participating in a seudat mitzvah (a meal marking the fulfillment of a mitzvah), such as a siyyum--a festive meal celebrating the conclusion of the study of a section of Torah). Most synagogues host such a celebration after morning prayers.