2,095. A Time Fixed Based on External Factors

Hilchos Issurei Biah 8:5

When a set time is established because of external factors, even if this happens several times, it isn’t a set time at all. This is because the period started because of that external cause. If a woman jumps and her period starts, and she subsequently jumps and her period starts again, she establishes a set time for that day without consideration for the fact that she jumped. For example, let’s say that she jumped on a Sunday and her period began. After 20 days, she jumped on Sunday and her period started again. Then, after 19 days, she jumped on Shabbos and her period didn’t start; rather, but it started after Shabbos without her jumping. The fact that this happened on three occasions establishes a set time for her on Sunday after 20 days, it being clear that the interval of time brings about her period rather than the jumping. The same rules apply in all similar cases.

Hilchos Issurei Biah 8:6

If a woman’s period starts on the 15th of one month, the 16th of the next month, the 17th of the month after that, and the 18th of the month after that, she establishes a set time that advances. If her period starts on the 17th in the fourth month, the set time is not established. Rather, she suspects the next month that her period will arrive on the same day as in that month. If it doesn’t, that day is purified and no longer suspected. Only a date that has been established three times need be invalidated by three occasions without her period arriving.