If a woman must immerse in a Mikveh on Friday night, should she immerse before or after the Shabbat meal?
A woman should preferably immerse before the Shabbat meal, for a number of reasons:
1) The preparations for Tevila (immersion) include "Chafifa," rinsing and combing one's hair, which a woman should do as close to her immersion as possible. "Chafifa" may not be done on Shabbat, and thus a woman who immerses on Friday night must perform "Chafifa" before Shabbat. Naturally, then, she must immerse as early as possible on Friday night, so as to minimize the lapse of time between the "Chafifa" and the immersion.
2) According to Sephardic practice, one may not bathe in hot water on Shabbat, even it had been heated before Shabbat. A woman who must immerse on Friday night must therefore either immerse in lukewarm water, or immerse in hot water during the period of Bein Ha'shemashot (twilight), which extends for approximately 13.5 minutes after sunset, during which bathing in hot water is permissible. Clearly, then, a woman who wishes to immerse in hot water must ensure to immerse during this period, rather than waiting until after the Shabbat meal.
3) For immersion to be valid, a woman must remove all dirt and other extraneous substances from the surface of her body and from her teeth. If a woman eats the Shabbat meal prior to immersion, there is a chance that particles of meat will become wedged in her teeth, which would disqualify her immersion.
Therefore, it is far preferable that a woman who must immerse on Friday night do so before the Shabbat meal, rather than afterward. If, however, she has no other option than immersing after the Shabbat meal, she may do so, on condition that she thoroughly cleans her teeth before immersing. (These laws apply to immersion on the night of Yom Tov, as well.)
See Halichot Olam, Helek 4, page 209.
