Chapter 8. The Laws of the Fast of the Firstborn

Who Fasts
A. Firstborns fast on the eve of Pesach, whether a firstborn through the father or a firstborn through the mother. [And even when Pesach falls on Shabbat, the firstborns fast on Friday [1] , and the fast continues until nightfall. When the eve of Pesach falls on Shabbat, they fast on Thursday, as explained below].

One Who Has a Headache
B. One who has pain in his head or eyes does not need to fast  [2] .

Accepting the Fast
C. The Fast of the Firstborn does not require acceptance from the previous day, because it is obligatory  [3] .

A Siyum Meal
D. The poskim disagree whether a siyum meal is effective to permit a firstborn to eat, and the custom today is to be lenient and eat by means of a siyum of a tractate  [4] . 

If He Did Not Complete It Himself
E. Even one who did not complete the tractate himself, but heard the siyum and participated in the siyum meal, is exempt from the fast  [5] .

If He Heard the Siyum
F. One who only heard the siyum but did not eat from the siyum meal appears to be exempt from the fast [and there is room for uncertainty regarding one who hears it by telephone].

Participated in the Meal
G. One who participated in the siyum meal but did not hear the siyum: it appears that one may be lenient and allow him to eat.

Women
H. Even among Sephardic communities, the custom is that women and girls do not fast  [6] .

[1] Mishnah Berurah (section 470, subsection 5).
[2]  Mishnah Berurah (there, subsection 20).
[3]  Like the law of a communal fast.
[4]  Mishnah Berurah (there, subsection 10).
[5]  Ibid.
[6]  Ben Ish Chai (Parashat Tzav).