9. Although only washing for the sake of pleasure ("ta'anug") is forbidden
[on Tish'ah B'av], a women whose ritual immersion is scheduled for the
night of Tish'ah B'av (that is, the night between the eighth and ninth of
Av) may not immerse herself, because sexual relations are forbidden [on
Tish'ah B'av] (1).
(1) Nowadays, the primary consequence of a woman immersing herself in the
mikvah after menstruating, is that she will then be permitted to have
marital relations with her husband. Since marital relations are forbidden
on Tish'ah B'av, her immersion in the mikvah on Tish'ah B'av will have no
immediate consequence, and thus it is forbidden, even though the woman
would be immersing in water for the sake of a mitzvah, and not for pleasure.