Chapter 122:3 and 4
Between the 17th of Tammuz and Tisha B'av
3. It is customary not to cut one's hair or shave during these days
[between the 17th of Tammuz and Tishah B'av] (1). Similarly, hair should
not be removed from any other portion of the body (2). Adults are forbidden
to cut children's hair (3).
4. If one's mustache interferes with one's eating, it appears to me that
one may be permitted to shave (or trim) it until the week during which
Tish'ah B'av falls. During that week, however, it is forbidden (4).
(1) Many Sefardic communities refrain from cutting hair and shaving, only
during the week in which Tishah B'av falls, and not from the 17th of
Tammuz, based on the ruling of the Shulchan Aruch (551:12).
(2) Both men and women are included in this prohibition against cutting
hair, although, according to the Mishna Berura (551:79), it might be
permissible ("efshar le'hatir..") for a woman to remove excess hair from
around her temples. A married woman or one of marriageable age, may shave
her legs even during the nine days between the first and ninth of Av (Rav
Moshe Feinstein, Z"tzl).
(3) The Mishna Berura (551:82) quotes the Chayei Odom who rules that an
adult is prohibited from cutting a child's hair ONLY during the week in
which Tishah B'av falls.
(4) According to the Mishna Berura (Sha'ar HaTziyun 551:90), one need not
be stringent with trimming a moustache that interferes with eating; in
other words, one may trim it even during the week in which Tishah B'av falls.