Three hairs tied together in a knot do not interpose, as three hairs cannot be tied so tightly that water cannot penetrate them. With regard to two hairs tied together in a knot, I do not know the halakha. And Rabbi Yoḥanan says: We have a tradition that only one hair interposes, but two or more do not prevent the water from reaching the body.

§ Rabbi Yitzḥak says: By Torah law, if there is an interposition between a person and the water, and it covers the majority of his body, and he is particular and wants the interposing substance removed, it interposes and invalidates an immersion in a ritual bath. But if it covers the majority of his body but he is not particular about that substance, it does not interpose. But the Sages issued a decree prohibiting substances covering the majority of his body with regard to which he is not particular, due to substances covering the majority of his body about which he is particular. And likewise, they issued a decree against substances covering the minority of his body with regard to which he is particular, due to substances covering the majority of his body about which he is particular.

The Gemara raises a difficulty: But let them also issue a decree prohibiting substances covering the minority of his body with regard to which he is not particular, due to substances covering the minority of his body about which he is particular. The Gemara answers: The Sages did not issue such a decree, as that prohibition is itself a rabbinic decree, and will we arise and issue a decree to prevent the violation of another decree?

§ The Gemara returns to discuss the issue of the correct time for immersion. Rav says: A menstruating woman who wishes to immerse to complete her purification process at her time, i.e., at the end of seven days, may immerse only at night, i.e., on the night after the seventh day, the night of the eighth day. But a woman who wishes to immerse not at her time, i.e., on the eighth day or afterward, may immerse either during the day or at night. Rabbi Yoḥanan says: Whether she is immersing at her time or not at her time, she may immerse only at night. This is because her daughter might follow her example when she is old enough, and also immerse in the daytime, but she may do so on the seventh day, when immersion is not valid.

And even Rav retracted his opinion and ruled that a woman who is immersing after menstruation may never do so during the day. As Rabbi Ḥiyya bar Ashi said that Rav said: A menstruating woman who wishes to immerse to complete her purification process, whether at her time or whether not at her time, may immerse only at night, because her daughter might follow her example and immerse during the daytime of the seventh day.

The Gemara cites examples of Sages who permitted immersion on the eighth day due to exceptional circumstances. Rav Idi decreed in Neresh that the women may immerse during the daytime on the eighth day due to the lions that roamed the area at night and posed a threat to women who immersed at that time. Rav Aḥa bar Yaakov decreed in Pappunya that the women should immerse during the daytime on the eighth day due to thieves who posed a threat to women who immersed at that night.

Similarly, Rav Yehuda decreed in Pumbedita that the women should immerse in the daytime on the eighth day due to the cold of the night. Rava decreed in Meḥoza that the women should immerse in the daytime on the eighth day due to the gatekeepers [abula’ei], who were untrustworthy and might hurt the women on their way to and from immersion.

§ Rav Pappa said to Rava and to Abaye: Since nowadays the Sages have given all women who menstruate the status of an uncertain greater zava, may they immerse on the seventh day during the daytime? A greater zava, after counting seven clean days, may immerse on the seventh day. Since all women who menstruate now count seven clean days after the bleeding ceases, more than seven days from the onset of her menstruating have passed by the seventh clean day.

Rava and Abaye replied that they may nevertheless not immerse on the seventh day, due to the statement of Rabbi Shimon. As it is taught in a baraita: The verse states: “But if she be purified of her ziva, then she shall count to herself seven days, and after that she shall be pure” (Leviticus 15:28). The purification of a zava must be after the seven days, i.e., after all of the days, which must be consecutive so that there are no days of impurity separating between the seven clean days. Rabbi Shimon says that the phrase “And after that she shall be pure” teaches that after the action of counting seven clean days she shall be pure. In other words, once she has examined herself at the beginning of the seventh day she may immerse on that day.

Rabbi Shimon continues: But the Sages said: It is prohibited to do so, i.e., to immerse on the seventh day, lest she come to a case of uncertainty. If she were to engage in intercourse with her husband on that seventh day after immersion, and afterward on the same day she experiences bleeding, it would retroactively nullify her entire seven clean days, which would mean that she engaged in intercourse with her husband while she was impure, rendering them both liable to receive the punishment of karet.

§ Rav Huna says: A woman may wash her hair on the first day of the week, Sunday, and immerse on the third day of the week, i.e., Monday night. The proof of this is that sometimes a woman washes her hair on the eve of Shabbat, but she does not immerse until the conclusion of Shabbat, which is the same interval as from Sunday to Monday night.

Similarly, a woman may wash her hair on the first day of the week, Sunday, and immerse on the fourth day of the week, Tuesday night. The proof of this is that sometimes a woman washes her hair on the eve of Shabbat, but she does not immerse until the conclusion of the Festival that occurs after Shabbat, which is the same interval as from Sunday to Tuesday night.

Furthermore, a woman may wash her hair on the first day of the week, Sunday, and immerse on the fifth day of the week, Wednesday night. The proof of this is that sometimes a woman washes her hair on the eve of Shabbat, but she does not immerse until the conclusion of two days of Rosh HaShana that occurs after Shabbat, which is the same interval as from Sunday to Wednesday night.

But Rav Ḥisda said: We say that all of these long gaps between a woman washing her hair and immersing are permitted. But we do not say as proof that since a woman washes her hair before Shabbat and immerses only several days later, she may do so during a regular week as well. One cannot learn this halakha from those situations involving Shabbat, as one cannot derive cases where alternatives are possible from those cases where alternatives are not possible. Where it is possible for a woman to wash her hair and immerse immediately, it is possible, and she is not allowed to do so any earlier. But where it is not possible for a woman to wash her hair immediately before immersion, e.g., when she must immerse after the conclusion of Shabbat or a festival, it is impossible, and for this reason she is permitted to wash her hair in advance.

And Rav Yeimar said: We even say the proof that since a woman washes her hair before Shabbat and immerses only several days later, she may do so during a regular week as well. Rav Yeimar agrees with Rav Huna in all cases apart from that of a woman who washes her hair on the first day of the week, Sunday, and immerses on the fifth day of the week, Wednesday night. He maintains that Rav Huna’s proof from the conclusion of two days of Rosh HaShana that is after Shabbat is not a sufficient proof, as even in such a case a woman may not wait so long. Rather, it is possible for her to wash her hair at night after the conclusion of Rosh HaShana and immerse on that same night, so that she should not have such a long gap between washing her hair and immersing.

Mareimar taught: The halakha is in accordance with the opinion of Rav Ḥisda, that a woman should not wash her hair many days before immersing except when there is no other possibility. And the halakha is also in accordance with the manner in which Rav Yeimar explains that if Rosh HaShana occurs after Shabbat a woman should wash her hair on the night of her immersion.

§ A dilemma was raised before the Sages: What is the halakha with regard to whether a woman may wash her hair at night and immerse on that same night? Mar Zutra deems it prohibited for her to do so. Since she will be in a hurry to immerse and return to her husband, there is a concern that she will not wash and examine her hair thoroughly. And Rav Ḥinnana from Sura deems it permitted for a woman to wash her hair on the night of her immersion.

Rav Adda said to Rav Ḥinnana from Sura: Was there not an incident like this involving the wife of Abba Mari, the Exilarch, who quarreled with her husband, and as a result she did not want to immerse in a ritual bath to become permitted to him; and Rav Naḥman bar Yitzḥak went to appease her and to persuade her to immerse, and she said to him: What is there that can be done now? There is no reason for me to hurry and immerse now.