Chapter 123:1&2
Erev Tish'ah B'av
1. When a "bris milah" (circumcision) or "pidyon ha'ben" (redemption of the first-born) occur on erev Tish'ah B'av (the day before Tish'ah B'av), one should hold the seuda (festive meal) before midday.
2. One should not go walking for pleasure on erev Tish'ah B'av (1). After midday (on erev Tish'ah B'av), it is the custom to study only [those parts of Torah] which one is permitted to study on Tish'ah B'av itself (2).
FOOTNOTES:
(1) Or engage in any other unnecessary pleasure activity which would divert one's mind from mourning.
(2) Torah learning generally engenders a state of joy, and therefore, on Tish'ah B'av, one is only allowed to learn those sections of the Torah which are appropriate to a time of mourning, such as those that deal with tragedy and destruction; for example, one may study the book of Job, and the sections of the Tanach and Talmud discussing the destruction of the Beit Hamikdash (Temple). One may also learn the laws of mourning.
There are many authorities who rule that only on Tish'ah B'av itself are we limited in terms of the sections of Torah we can study, however on erev Tish'ah B'av, one may learn all sections of Torah (Mishna Berura 553:8)
