Back in Mitzvah #181, we discussed the nature of menstrual impurity. Really, it’s just one of many kinds of ritual impurity, some of which were unique to women or men, but most of which were gender-neutral. There, we said that the reason this impurity – called niddah impurity – is still practiced today is that it has a practical application. Most forms of ritual uncleanliness would only prevent a person from entering the Temple or eating from sacrifices. Niddah impurity, however, prohibits marital relations for the duration, which makes an impact even in the absence of the Temple. The prohibition on such relations is this mitzvah.
The Torah defines the niddah period as seven days until the woman can attend the mikvah and be purified. If a woman does not immerse in a mikvah, she remains in this state until she does so and sexual relations are Biblically prohibited.
Much has been written about the benefits of a couple observing the laws of taharas hamishpacha (“family purity”) by refraining from marital intimacy during this period each month. Sex is a gift that God wants people to enjoy, but as with food, He doesn’t want us to be animals about it or slaves to our passions. Accordingly, He contextualized a way for people to enjoy these pleasures. When it comes to sex, the niddah period is an enforced break. During this time, the couple focuses on other aspects of their relationship. When they may resume physical intimacy, after the woman attends the mikvah, they have a new sense of appreciation for it.
This mitzvah applies in all times and places. It is the subject of the Talmudic tractates of Niddah. It is codified in the Shulchan Aruch in Yoreh De’ah 183-200. This prohibition is #346 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #132 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.