When Tisha Be’Av is a נדחה does it still push off a Leil Tevilah, or not?
If the tevilah is on Friday Night, the lady should go to the mikva, and it is permitted to be together on Friday Night.
If the tevilah is on Motza’ei Shabbos, it is pushed off to Sunday night.
The Shulchan Aruch (554:19) rules that on Shabbos of Tisha Be’Av even tashmish ha-mitah is permitted. The Rema rules that one must refrain from marital relations, but the Mishnah Berurah (40) writes that in a case of a leil tevilah one relies on the opinion of the Shulchan Aruch.
This, however, applies only to the Shabbos. On Sunday, when the fast of Tisha Be’Av is practiced, carries all the halachos of Tisha Be’Av, including the prohibition on marital relations.