The sunrise to sunset ta’anit (fast) bekhorim (of the firstborn) is the only fast that applies to just a segment of the community: all males who are the firstborn children in their families (if the firstborn child is female, the first son born after her is not obligated). The father of a child too young to fast fasts for him, and if he himself is bekhor, the mother fasts for the child on the day of ErevPesach [the day in which Passover begins at nightfall]. Since it is forbidden to abstain from eating on Shabbat (except for Yom Kippur), when Erev Pesach falls on Saturday night, the fast takes place on Thursday.
There is a widely practiced exemption: On the principle that fasting is prohibited on a joyous occasion, Judaism allows for anyone who attends a religious feast to forego fasting. It is customary to hold a celebratory meal on the completion of study of a tractate of Talmud, called a siyyum (conclusion). So rabbis initiated the practice of studying a portion of a Talmud tractate after morning services, held especially early on Erev Pesach. All the firstborn are invited to be present for the conclusion and share cake and schnapps afterward, considered a seudat (meal) mitzvah (in honor of a commandment; in this case, studying the Torah).
Among some Sephardim [Jews of Mediterranean descent], women used to observe the fast of the firstborn. The Syrians, who stringently observe it, include their women in the siyyum and seudat mitzvah following morning services. As an alternative, a community would sometimes arrange for the poor to be married on the day of Erev Pesach. The firstborn were invited, since the wedding meal is a seudat mitzvah exempting them from the fast.
Excerpted from Celebrate! The Complete Jewish Holiday Handbook (Jason Aronson Inc.).
Pronounced: shuh-BAHT or shah-BAHT, Origin: Hebrew, the Sabbath, from sundown Friday to sundown Saturday.
Pronounced: TALL-mud, Origin: Hebrew, the set of teachings and commentaries on the Torah that form the basis for Jewish law. Comprised of the Mishnah and the Gemara, it contains the opinions of thousands of rabbis from different periods in Jewish history.