Вайикра 15:28 Галаха: Mishneh Torah, Positive Mitzvot, Arukh HaShulchan & Sefer HaChinukh

That a woman having an issue shall bring a sacrifice, after she is cleansed of her issue, as it is said, “But if she be cleansed of her issue….” (Lev. 15:28-30).

9. Regarding the laws of a zavah, she is permitted to immerse even on the seventh day, and there is no need to wait until nightfall. Although if she sees blood until nightfall, she conceals everyone who is clean, as mentioned in Siman (Chapter) 196. However, from the perspective of immersion, it is allowed to immerse, as it is written: "And she shall count for herself seven days, and after that, she may be purified" (Leviticus 15:28). Once her counting is completed, she becomes pure [Rashi, Pesachim 90b]. On the seventh day, her counting is complete. If she truly sees blood, she conceals, and both the counting and purification are nullified. Thus, she is as if she did not immerse, and she needs other clean individuals and another immersion. However, for a Niddah, her immersion is nothing until nightfall. After the seven days of her menstruation, as it is written: "For seven days, she shall be in her menstrual impurity; and on the eighth day, she shall be clean" (Leviticus 15:19). It is meant to be understood that throughout these seven days until their completion, she is in her menstrual impurity. If she immerses, her immersion is nothing, and, therefore, she must necessarily immerse at night after the seven days of her menstruation. Regarding a woman who has given birth, she is compared to a Niddah. As it is written: "Like the days of her menstrual impurity, she shall be impure" (Leviticus 12:2). This applies only in her designated time. However, if it is not in her designated time, for example, on the day after the eighth day, she may immerse during the day or night, as the seven days of her impurity have already concluded [Siman 196, Shulchan Aruch].

And we are not so stringent with one who finds a stain to require her a day of a break of purity besides the day upon which she found the stain, as we said about one who [experienced blood] only one day. Rather, certainly since she checked herself after she found the stain and found it pure, she counts seven clean ones besides that day that she found the stain. [This is] meaning to say that she counts her tally from the morrow of the day upon which she found the stain, like the law of a woman who [experienced] blood two or three days, as we said above. From the outset, a woman needs an examination on each and every day of all seven days (Niddah 68b). But ex post facto, if she only checked on the first day — which is the morrow of the day she stopped [bleeding] — and she did not check again, even if several days after the seven passed, she checks herself at the time of immersion; and that is enough for her, as she already had a pause of purity before then. [If] she did not check herself on the first day, but rather only on the day of her stopping alone (the day before the first day), she checks herself on the seventh day; and that is enough for her, and [so] she immerses on the night of the eighth. [If] she did not check herself, neither on the first day nor on the seventh day — even if she checked herself on the night of the eighth — even though she made a pause of purity before then (before the first day), those seven days that passed do not count for her at all. As she did not check during them, not at the beginning and not at the end, and she needs to count seven clean days from the time that she checked. And it is possible that the essence of this matter is because the Torah commanded (Leviticus 15:28), “and she shall count for herself [...] and she shall become pure.” And since she did not check — not at the beginning and not at the end — there is no counting here.