The mishna is referring to a case where she discharges ritually impure blood in the midst of discharging ritually pure blood. For example, if she normally first discharges blood that is not red, and it therefore does not render her impure, and then she experiences a discharge of red blood.

The mishna teaches: Or a type of feverish shuddering overtakes her, and likewise the same applies with regard to any sensation of the like. The Gemara asks: What is added by this last phrase? Rabba bar Ulla said: It serves to include a woman whose head is heavy upon her or her limbs are heavy upon her, or she trembles or belches constantly.

Rav Huna bar Ḥiyya says that Shmuel says: The Sages said, with regard to establishing a set period of days, that two days are sufficient, i.e., if a woman experiences bleeding twice on the same date of the month or after the same interval, she has a fixed menstrual cycle. By contrast, with regard to a fixed menstrual cycle based on a physical sensation, it is sufficient if she experiences bleeding even once. But for those sensations which the Sages did not include in the mishna, she has a fixed cycle only if she experiences bleeding three times accompanied by one of those symptoms.

The Gemara asks: When Shmuel said: For those which the Sages did not include, what did he intend to add? Rav Yosef says: This serves to add the case mentioned above, i.e., her head is heavy upon her, or her limbs are heavy upon her, or she trembles or belches constantly. Abaye said to Rav Yosef: What is that addition teaching us? In effect, it is already taught in the mishna, as Rabba bar Ulla explained above that the additional phrase: The same applies with regard to any sensation of the like, serves to include those sensations. Rather, Abaye said: Shmuel’s phrase serves to add a case where she ate garlic and saw menstrual blood, or ate onions and saw menstrual blood, or chewed pepper and saw menstrual blood, i.e., these triggers can give rise to a fixed cycle, but only after three occurrences.

Rav Yosef said: I did not hear this tradition concerning the opinion of Shmuel, that with regard to days a cycle is fixed if she experiences bleeding twice, whereas for physical sensations a cycle is fixed after one occurrence. Rav Yosef had fallen ill and forgotten his studies and was therefore unable to remember that such a ruling had been issued.

His student, Abaye, said to him: You yourself told us this halakha, and it was with regard to this following matter that you told this halakha to us, taught in the mishna below: If a woman was accustomed to see the flow of blood on the fifteenth day and she deviated from the norm to see the flow of blood on the twentieth day, then on both this day, the fifteenth, and that day, the twentieth, it is prohibited for her to engage in intercourse. If she deviated from the norm to see the flow of blood on the twentieth day three times, it becomes permitted for her to engage in intercourse on the fifteenth day, and she has established the twentieth day for herself as the day of her fixed menstrual cycle, as a woman establishes a fixed menstrual cycle only after she establishes it three times.

Abaye continues: And you, Rav Yosef, said to us with regard to this mishna that Rav Yehuda said that Shmuel said: This is the statement of Rabban Gamliel bar Rabbi Yehuda HaNasi, who said it in the name of Rabban Shimon ben Gamliel. But the Rabbis say: If she saw a discharge of blood even once, she has a fixed cycle and does not need to repeat and experience bleeding a second time or to repeat a third time.

And we said to you: Since you said to us that she does not need to repeat a second time, is it necessary for you to mention that she does not need to repeat a third time? And you said to us that when you said that she does not need to repeat a second time, this is referring to a fixed menstrual cycle based on a physical sensation, whereas when you said she does not need to repeat a third time, that is referring to a fixed cycle of days.

The Gemara asks: But why did Shmuel have to explain that the mishna is the statement of Rabban Gamliel bar Rabbi Yehuda HaNasi? Let him say simply that this mishna is the statement of Rabban Shimon ben Gamliel, who always requires repetition three times to establish a presumptive status. The Gemara answers that this is what Shmuel teaches us: That Rabban Gamliel, son of Rabbi Yehuda HaNasi, holds in accordance with the opinion of Rabban Shimon ben Gamliel.

MISHNA: If a woman was accustomed to see the flow of blood at the beginning of the sensation that accompanies her fixed cycle, even if on one occasion she happened to experience bleeding only at the end of the sensation, all the ritually pure items that she handled within the duration of that sensation that accompanies her fixed cycle are ritually impure. If the woman was accustomed to experience bleeding at the end of the sensation that accompanies her fixed cycle, all the ritually pure items that she handled within the duration of that sensation that accompanies her fixed cycle are pure.

Rabbi Yosei says: Even specific days and specific hours determine a fixed menstrual cycle: If a woman was accustomed to see the flow of blood on a certain day of the month at sunrise, it is prohibited for her to engage in intercourse with her husband only at sunrise; but during the night before and the following day, it is permitted for her to engage in intercourse. Rabbi Yehuda says: Once sunrise passed and she did not menstruate, the entire day is hers and she may engage in intercourse, in accordance with the opinion of Rabbi Yosei; but contrary to the opinion of Rabbi Yosei, intercourse the night before is prohibited.

GEMARA: A tanna taught in a baraita: In what case did Rabbi Yosei say that specific days and hours determine a fixed menstrual cycle? For example, if a woman was accustomed to see the flow of blood on the twentieth day and again on the twentieth day, and in the sixth hour of the day and again in the sixth hour of the day the next time; if the twentieth day arrives a third time and she does not see a flow of blood, it is prohibited for her to engage in intercourse for the entirety of the first six hours of the day; this is the statement of Rabbi Yehuda. Rabbi Yosei deems it permitted for her to engage in intercourse until the sixth hour of the day, and says that she must be concerned only during the sixth hour.

The baraita continues: If the sixth hour passed and she did not see a flow of blood, it is prohibited for her to engage in intercourse for the entirety of the day; this is the statement of Rabbi Yehuda. And Rabbi Yosei deems it permitted for her to engage in intercourse from minḥa time onward, i.e., from the beginning of the seventh hour.

§ The mishna teaches with regard to a woman who was accustomed to see the flow of blood on a certain day of the month at sunrise, that Rabbi Yehuda says: Once sunrise passed and she did not menstruate, the entire day is hers and she may engage in intercourse, but intercourse is prohibited the night before. The Gemara objects: But isn’t it taught in a baraita that Rabbi Yehuda says: The entire night is hers? This indicates that she may engage in intercourse during the night.

The Gemara explains that it is not difficult. Here, when it is permitted for her to engage in intercourse the entire night, it is referring to a case where she is accustomed to see a flow of blood at the beginning of the day, i.e., immediately after sunrise. And there, in the mishna, which deems it prohibited for her to engage in intercourse all night, it is referring to a case where she is accustomed to see a flow of blood at the end of the night, i.e., just before sunrise.

The Gemara cites further baraitot that deal with Rabbi Yehuda’s opinion. It is taught in one baraita: Rabbi Yehuda deems it prohibited for her to engage in intercourse before the time of her fixed cycle, but deems it permitted for her to do so after the time of her fixed cycle if she did not experience bleeding then. And it is taught in another baraita: Rabbi Yehuda deems it prohibited for her to engage in intercourse after the time of her fixed cycle but deems it permitted for her to do so before the time of her fixed cycle.

The Gemara resolves this apparent contradiction: And this is not difficult; here, the baraita that rules that it is prohibited for her to engage in intercourse before the time of her fixed cycle is referring to a case where she is accustomed to see a flow of blood at the end of the night. By contrast, there, the baraita that states that it is prohibited for her to do so after the time of her fixed cycle is referring to a case where she is accustomed to see a flow of blood at the beginning of the day.

Rava said: The halakha is in accordance with the opinion of Rabbi Yehuda. The Gemara asks: And did Rava actually say this? But isn’t it taught in a baraita: The verse states with regard to a menstruating woman: “You shall separate the children of Israel from their impurity” (Leviticus 15:31). From here, Rabbi Yirmeya said that there is a warning to the children of Israel that they should separate from their wives close to the time of their menstrual cycles.

And how close? Rava says: One twelve-hour period, i.e., either from sunrise to sunset, or from sunset to sunrise. The Gemara explains the difficulty: What, is it not referring to an additional twelve-hour period, i.e., the prohibition applies to the time of her expected period itself, either day or night, as well as the previous twelve hours? This is not in accordance with the opinion of Rabbi Yehuda, who deems it prohibited for her to engage in intercourse only for the twelve hours of the expected time of her period. The Gemara answers: No, it is referring to that same twelve-hour period.

The Gemara asks: And why do I need two statements of Rava, i.e., both that the halakha is in accordance with the opinion of Rabbi Yehuda and that a woman must separate for only one twelve-hour period? The Gemara answers that both are necessary, because if Rava had taught us only this, that the halakha is in accordance with the opinion of Rabbi Yehuda in the mishna, I would say that this statement applies only to the handling of pure items, but with regard to engaging in intercourse with her husband, it does not apply, and with regard to intercourse, the halakha is in accordance with the opinion of Rabbi Yosei. Therefore, Rava teaches us that even with regard to intercourse, the halakha is in accordance with the opinion of Rabbi Yehuda.

And by contrast, if Rava had taught us only that statement, that she must wait a twelve-hour period, I would say that she must separate close to the fixed time of her cycle for an additional twelve-hour period, as suggested above. Therefore, Rava teaches us that the halakha is in accordance with the opinion of Rabbi Yehuda, which indicates that she separates for only that single twelve-hour period.

MISHNA: If the woman was accustomed to see the flow of blood on the fifteenth day and she deviated from the norm to see the flow of blood on the twentieth day, then on both this day, the fifteenth, and that day, the twentieth, it is prohibited for her to engage in intercourse. If she deviated from the norm twice, then on both this day, the fifteenth, and that day, the twentieth, it is likewise prohibited for her to engage in intercourse.

If she deviated from the norm to see the flow of blood on the twentieth day three times, it becomes permitted for her to engage in intercourse on the fifteenth, and she has established the twentieth day for herself as the day of her fixed menstrual cycle, as a woman establishes a fixed menstrual cycle only after she establishes it three times. And a woman is purified from the existing fixed menstrual cycle, in the sense that intercourse is permitted on that day, only when she has been displaced from that day three times.