2,097. Establishing a Veses During Niddah or Zivah Days
Hilchos Issurei Biah 8:9
A woman doesn’t establish a fixed time during her niddah days when she has menstruated; since she menstruates on one of these days, she can’t fix a time on any of those seven days. Similarly, a woman can’t establish a fixed time in her eleven zivah days. She may, however, establish a fixed time on one of her niddah days when she hasn’t menstruated. If she established a fixed time in her zivah days, she must be concerned for it. When a fixed time has been established in a woman’s zivah days, it is overturned if it is fails to appear even once; being missed three times is unnecessary. This is because of the assumption that a woman’s blood is held back on these days.
Hilchos Issurei Biah 8:10
To show concern for this fixed time (in the previous halacha) means that if she saw blood at this time for even one day, she must wait like a niddah out of doubt. She is not permitted to engage in marital intimacy on that day even if she didn’t see blood just like on other days of fixed times. If she sees blood for three consecutive days, then she’s a zavah.