הלכה אויף ויקרא 15:31

That He prohibited any impure person from entering the Sanctuary (the Tabernacle) - and everything that is similar to it for [all] the generations: All of the courtyard and from the Gate of Nikanor onwards, which is the courtyard of the Israelites. And that is His, may He be exalted, saying, "so that they shall not render their camp impure" (Numbers 5:3). And in the Gemara, Makkot (Makkot 14b), they said, "One who entered the Temple [while] impure - a punishment is written and a prohibition is written: The punishment - 'he has rendered impure the Tabernacle of the Lord, and that soul shall be excised' (Numbers 19:13). The prohibition - 'so that they shall not render their camp impure.'" And in the Mekhilta: "'Command the Children of Israel to send away from the camp' - is with a positive commandment. But from where [do we know] it is [also] with a negative commandment? You can say, 'so that they shall not render their camp impure.'" And the prohibition about this content has already been repeated with different language; and that His, may He be exalted, saying about a woman who has given birth, "and she shall not come into the sanctuary" (Leviticus 12:4). And in the [Sifra] (Sifra, Tazria Parashat Yoledet, Section 1:1), they said, "Because it is stated (Leviticus 15:31), 'And you shall separate the Children of Israel,' I would understand, whether from its front or from its back, and one is impure, he is liable. [Hence] we learn to say, 'and she shall not come into the sanctuary.'"And there it is explained that the law of a woman who has given birth and the law of other impure people is the same with regards to this. And they [also] said in the [Sifra] (Sifra, Acharei Mot, Chapter 12:13-14), about His saying "But if he does not wash and does not bathe his body" (Leviticus 17:16), "How is this? For [failure in] the bathing of his body, the punishment is excision, but for [failure in] the washing of his clothes, it is with forty [lashes]. And from where [do we know] that it is speaking here of his rendering the sanctuary and its consecrated objects impure? It prohibited and punished, etc." Behold it has been made clear that one who transgresses this negative commandment - if he was intentional, he is punished with excision; and if he was inadvertent, he must bring a variable offering, as we explained in Commandment 72 of the Positive Commandments. And the regulations of this commandment have already been explained at the beginning of Shevuot and in Horayot. (See Parashat Nasso; Mishneh Torah, Admission into the Sanctuary 3.)

The Rabbis expounded¹Maseches Shavuos 18b. (on the verse) "You shall separate the Children of Yisrael from their impurities."²Leviticus 15:31. Rabbi Josiah said: "This verse is an admonition to the Children of Yisrael, that they must separate from their wives prior to their expected periods." How long before? Rava said,³Maseches Avodah Zara 78a. "An onah." An onah is either a day or a night. If she is accustomed to perceive the flow during the day, even though she is accustomed to perceive this flow at the end of the day, she is nevertheless forbidden to her husband from the beginning of the day. Likewise, if she is accustomed to perceive in the morning and has not yet perceived, nevertheless, she is forbidden the entire day until the night.⁴Most poskim agree that the length of each onah depends on the length of the day or night at that time and place (Beis Yosef, Taz, Toras Hashelomim and Placey). Chochmas Adam rules that it is no less than twelve hours at any time. Likewise, if her periods occur at night, even if she is accustomed to perceive the flow in the beginning of the night, or if she is accustomed to perceive the flow at the end of the night, nevertheless she is forbidden to her husband the entire night. According to many authorities,⁵Trumas Hadeshen Responsa 250. not only marital relations are forbidden at this time, but also every other type of intimacy is forbidden, and it is proper to heed this opinion.⁶Bach, Shach, and Toras Hashlomim say that one who is strict in this will be blessed; but from the Halachic standpoint it is permitted. (Shulchan Aruch, Ramah, Bach, Radvaz etc.)

And the laws of the measure of the stain (Niddah 58b), and in which case it is attributed to a stain [from something else] and in which case it is not attributed [to it]. And one who has a wound in that place, if she attributes the blood to it until the wound heals. And the matter of what is the law of a girl minor who has had intercourse and has blood flow from her (Niddah 64b). And the blood of virginity (breaking the hymen), that the Sages determined to be like menstrual blood (Niddah 65b). And the many laws of the regular times, and the law of separation before her regular time, which is the [menstrual] period — meaning to say that day or that night that she is accustomed to [experiencing blood] upon it — is forbidden for sexual relations. [This is] from that which it is written (Leviticus 15:31), “And you shall separate the Children of Israel from their impurity,” and the explanation came that they should separate from their wives before their regular times. But after the period of the regular time has passed [without blood], they are permitted in sexual relations. And the matter of the distancing from the wife that they, may their memory be blessed, commanded (Shabbat 11a) regarding the matter of menstruation, in order that a man not stumble in sin — and it is that a man should not eat with his menstruant wife from the same tray, nor pass a thing from his hand to her hand (Shabbat 13a), nor speak to her about things that often cause sin nor similar to these things.