Family purity טהרת המשפחה

The laws of purity and impurity take up a whole order in the Torah, and nearly all of them ceased with the Temple: with no ashes of the red heifer there is no purification. One remained: niddah. It remained because it belongs not to the Temple but to the house.

How it works. From the beginning of the menses to their end and seven "clean" days after, husband and wife do not touch one another; then the wife immerses in a mikveh, and they come together again. The seven days beyond what the Torah says are a custom the daughters of Israel took upon themselves, which became law.

Why a community built the mikveh first. Not the synagogue: one may pray at home and study from a book, but there is nowhere else to immerse. So in the statutes of a kahal the mikveh comes before much else, and archaeologists find ancient immersion pools wherever Jews lived—in Jerusalem, at Masada, at Gamla, under the floors of the houses of the anusim in Spain.

And a thing worth saying. Of all Jewish life this is the most private part, and the least written of in chronicles: the law was kept or not kept in silence. The more notable, then, that women learned it from women, and that the first books written in Yiddish for women are about this.