Since G-d decided that our redemption required taking the first born of Egypt, the first born of Israel are required to fast the day before Pesach (except when the holiday begins on Saturday night, since we don't fast on Shabbat, in which case the fast is the preceding Thursday).  However, minor fasts like this can be broken if partaking in a celebratory meal.  One way to have such a meal is a siyyum, or joining the concluding study of a larger body of rabbinic literature.  It is a longstanding tradition for first-born sons to come together that morning for such a siyyum where we first borns will break the fast together.