Chapter 122:5 Between the 17th of Tammuz and Tisha B'av
5. Cutting one's nails is not forbidden until the week during which
Tish'ah B'av falls. A woman who must cut her nails in preparation for her
ritual immersion may do so [during this time period] (1). Similarly, a
"mohel" (one who performs a circumcision) may trim his nails in preparation
to perform the "pri'ah" (2).
(1) Since there is a dispute among the "acharonim" (later authorities) as
to whether cutting nails is prohibited at all, we are lenient when a
mitzvah is involved. Therefore, if Tishah B'av falls on Shabbos, cutting
nails is permissible on Friday for the sake of honoring Shabbos.
(2) "Priah" is one of the steps involved in circumcision.