2,087. Four Nights Out of Eighteen

Hilchos Issurei Biah 7:13

We have discussed how when a woman has niddah bleeding for seven days, she is permitted to engage in marital intimacy on the night of the eighth day after immersing in a mikvah. A minor zavah who watches one clean day and immerses in a mikvah is likewise permitted to engage in intimacy that night. A major zavah must count seven clean days, after which she immerses in a mikvah and may engage in intimacy on the night of the eighth day. There are only 11 days between one set of niddah days and the next. During these 11 days, a woman would be rendered either a minor or a major zavah.

Hilchos Issurei Biah 7:14

When you remember all the underlying principles, you’ll understand why the Sages said that a woman who has a fixed pattern in which she sees blood on one day and not on the next may engage in intimacy on the night and day of the eighth day, which is the first day after her niddah days. She may engage in relations on only four nights of the 18 days; she may not be intimate on the pure days because these days must be watched corresponding to her impure days. Therefore, if on her impure days she always finds blood at the start of the night, she may only engage in intimacy on the eighth day, which is the first day after her niddah days.