The Halachot of Nidda require a woman to count seven "clean days," during which no bleeding occurs, after which she may immerse in a Mikveh to once again become permissible to her husband. In an earlier edition of Daily Halacha, we noted that the immersion must take place after sundown at the conclusion of the seventh day. Even if it is more convenient for the woman to immerse before sundown, such as if she has plans or will be traveling after sundown, she may not do so. It is only in a "Sha’at Ha’dahak" – under extenuating circumstances – that Halacha permits a woman to immerse before sundown on the seventh day. A competent Rabbi must be consulted for guidance whenever such situations arise, as one may not necessarily assume that her situation qualifies as a "Sha’at Ha’dahak." It goes without saying that a Mikveh must not establish a policy of facilitating immersions before sundown for the sake of convenience, to allow the Mikveh staff to return home early in the evening.
The question arises as to whether a woman may immerse during the following day. If, for whatever reason, a woman did not go to the Mikveh on the night after the seventh day, and she wishes to immerse the following day, must she wait until after sundown, or may she immerse already during the day? One might argue that this case differs from that of immersion on the seventh day, when the required seven-day waiting period had yet to be completed. On the eighth day, it would seem, since the required period has passed, it should be permissible to immerse even before sundown.
In truth, however, this is not the case. The Sages enacted a provision requiring a woman to wait until after sundown even in such a case, in order to avoid misconceptions. They were concerned that a daughter might see her mother go to the Mikveh on the eighth day and mistakenly assume that it is the seventh day. She would then be under the impression that immersion is allowed on the seventh day. The Sages therefore required waiting until after sundown even on the eighth day, and they applied this provision even if the woman has no daughters and there is nobody else around who might make this mistake.
However, in this case, as opposed to that of the seventh day, there is considerable room for leniency. For example, if a woman did not immerse on the night after the seventh day due to circumstances beyond her control, such as if she was sick, or there was a power failure and the Mikveh could not operate, then she may immerse during the day on the eighth day. Another example is a case where on the night after the seventh day the woman cannot immerse without other people knowing about it, and she wants to keep this information private. In all these cases, Halacha permits the woman to immerse even before sundown on the eighth day.
The Rama (Rav Moshe Isserles of Cracow, 1525-1572) ruled that in such a case, when Halacha permits a woman to immerse before sundown on the eighth day, she should wait until nightfall before informing her husband that she immersed. Maran (in Bet Yosef) disputes this ruling. Hacham Ovadia Yosef, in Taharat Ha’bayit (vol. 2, p. 492) rules that one may abide by the Rama’s stringency as a personal measure of piety ("Midat Hasidut"), but this is certainly not required, as Halacha follows the view of the Bet Yosef.
Summary: If, for whatever reason, a woman did not immerse in a Mikveh on the night after the seventh of the seven "clean days," she immerses after sundown on the eighth day. However, in situations where she was unable to immerse the night before due to circumstances beyond her control, such as if she was sick, she may immerse during the day.