2,072. How to Count Days

Hilchos Issurei Biah 6:6

Throughout a woman’s life, starting from when she establishes a time when she expects to menstruate and lasting until she dies or until establishes a different time when she expects to menstruate, she should count seven days from the start of the day when she expects to menstruate plus eleven days after them. She then counts another seven days plus eleven days. Pay careful attention and one will know if a woman discovers blood whether it was during the days of niddah or the days of zivah. Throughout a woman’s life, she will have seven days of niddah followed by eleven days of zivah. The exception is if the pattern is interrupted by childbirth, which we will address IY”H in the next chapter.

Hilchos Issurei Biah 6:7

If a woman finds uterine blood during her days of zivah and it lasts only one day or two consecutive days, she is called a minor zavah and “one who watches day for day.” If she found uterine blood on three consecutive days, she is a zavah in the fullest sense of the term; such a woman is called a major zavah or simply a zavah without elaboration. This is based on Leviticus 15:25, “If a woman has a flow of blood for many days....” The minimum implied by “days” is two; adding “many” suggests a minimum of three days.