Tevilah in St. Petersburg
In our city St. Petersburg , Russia, in the summer tzeis ha-kochovim (Gra) is at 12:30 at night, because of the “white night.” Can a woman rely on the opinion of Rabbeinu Tam and immerse in mikveh (on 7 day) before tzeis, and come home after? It is very difficult to immerse at 12:30 at night and come home; this can be at 2 at night on the leil tevilah.
Under extenuating circumstances, where it is difficult to wait until tzeis ha-kochaviim, it is permitted to immerse in the mikvah after sunset, even in the Bein Ha-Shmashos period.
It is not permitted, however, to rely on the opinion of Rabbeinu Tam and immerse during the day itself, before sunset.
Immersion at night on the seventh day is a rabbinic enactment, and therefore in principle one can immerse even in the Bein Ha-Shmashos period (see Yoreh De’ah 197; Shach 11; Pischei Teshuvah 13).
If somebody follows the rulings of Rabbeinu Tam (concerning the time of night) even for leniencies, the Bein Ha-Shmashos time will begin only later. But for the vast majority of people, for who the Rabbeinu Tam times might only be a chumra, one can assume that the Bein Ha-Shmashos time begins as soon as the sun has completely set.
[See also Teshuvos Ve-Hanhagos Vol. 5, no. 267, concerning those who follow Rabbeinu Tam for all purposes.]
Therefore, under the difficult circumstances of St. Petersburg’s “white summer,” it is permitted to immerse as soon as the sun has set.
However, it is not permitted to immerse during the day itself. Although the opinion of Rabbeinu Tam is that one can immerse on the seventh day, the halachah is that one must wait until night – though the Rambam (Issurei Bi’ah 11:17) and ShulchanAruch (197:3) write that immersion is effective on a bedieved level.
Only in cases of absolute oness do we find Poskim permitting immersion in the day itself (see Shevet Ha-Levi Vol. 3, no. 170), and although the circumstances of our case are difficult, it is not similar to the cases of oness gamur mentioned by Poskim (see Sidrei Taharah 13; Pischei Teshuvah 11; a commonly mentioned case is where the couple has to go away for several weeks [but not for several days] immediately on the night after the seventh day, so that there is no time for immersion at night – see Otzros Ha-Taharah, Tevilah no. 69).
Note that where a woman immerses in the mikvah before nightfall, she should not be in a state of yichud with her husband until nightfall.
This answer was given after consultation with HaRavWeissshlita.