According to the accepted custom, women are not required to observe the Ta’anit Bechorot (Fast of the Firstborn) on Ereb Pesah, or participate in a Siyum to absolve themselves from the fast. Nevertheless, there are some firstborn women who have the practice of refraining from eating until they receive some cake from the Siyum made in the synagogue for the male firstborn (which some people sarcastically call "the magic cake"). Hacham Bension Abba Shaul (Israel, 1923-1998) writes that although a man who is a firstborn must actually attend a Siyum, and simply being brought a piece of cake does not absolve him from the fast, nevertheless, firstborn women may adopt this practice, since in any event they are not required to fast.
In order for a firstborn to absolve himself from the fast, he must either eat a Ke’zayit of Mezonot food (approximately 30 grams) or drink a Rebi’it of wine or grape juice (approximately 3.2 ounces) at the Siyum celebration. It occasionally happens that cake made from potato starch is served at the Siyum since the premises have already been cleaned for Pesah. One does not fulfill requirement with such cake, since it is not a Mezonot food.
If a firstborn attends a Berit or Pidyon Ha’ben on Ereb Pesah, and he either eats or drinks wine, this suffices to absolve him from the fast. This applies also to a Bar Misva celebration held on the day the boy becomes a Bar Misva. When a Berit is held on Ereb Pesah, the father, the Mohel and the Sandak are all exempt from the fast, as it is considered a holiday for them.
Firstborns must ensure not to eat or drink anything until after the Siyum. Even if one knows that he will be attending a Siyum, he may not eat or drink until that point.
If a firstborn hears a Siyum before he prays, such as if a Siyum is held at an early Minyan and he will be going to a later Minyan, then he should drink a Rebi’it of water at the Siyum (as drinking water is permissible before praying in the morning), and then take some cake or wine from the Siyum to eat or drink after he prays Shaharit. This is the ruling of Hacham Bension Abba Shaul.
If a firstborn does not have any Siyumim taking place in his area, and he cannot study a complete Masechet of Talmud, then he may make a Siyum on completing a Masechet of Mishna with the commentary of Rabbenu Ovadia Mi’Bartenura. This is the ruling of Hacham Ovadia Yosef.
One who makes a Siyum on a Masechet does not have to learn the Masechet in order. It is perfectly acceptable, for example, to study all the chapters except the fifth, and then complete the fifth chapter at the Siyum.
Hacham Bension Abba Shaul notes that the practice of attending a Siyum to absolve oneself from the fast has no mention in the writings of the Rishonim (Medieval scholars), and was instituted later, during the period of Aharonim. This was done because people did not have the strength to fast and then conduct the Seder properly that night. Therefore, Hacham Bension writes that if somebody knows he fasts well and will have no trouble abstaining from food and drink the entire day and then properly conducting the Seder, it is preferable for him to fast. We should note, however, that this can be very difficult, especially when Daylight Savings Time begins before Pesah. The Seder generally does not begin before 8:30, and one who fasts on Ereb Pesah will be drinking two cups of wine on an empty stomach without eating anything substantial until around 10pm or so. This would be very difficult for the vast majority of people, and therefore it is preferable to attend a Siyum unless one is confident that fasting will not affect him at the Seder that night.
The Ta’anit Bechorot fast was instituted to commemorate the plague of the firstborn which struck Egypt on the night of the Exodus, from which the firstborn of Beneh Yisrael were saved. The question naturally arises as to why this miracle would be commemorated by fasting. On Purim, for example, we were saved from death and we therefore celebrate with a festive meal. Why would the firstborn commemorate being rescued from the plague by fasting?
Hacham Bension explains that when a person is the beneficiary of a miracle, the miracle causes his merits to diminish. It comes out of the person’s "account," so-to-speak, and his merits therefore need to be replenished, which the firstborn try to do through the fast of the firstborn. Therefore, one who is able to actually fast should do so, though, as mentioned, only if he is confident that it will not hamper his ability to properly conduct the Seder.