2,094. Setting and Canceling a Designated Time
Hilchos Issurei Biah 8:3
As already discussed, any woman who doesn’t have a set time when her period is expected may not engage in marital intimacy without first making an internal examination. If she has a set time, she is prohibited to engage in intimacy for the entire duration of that time. If her set time is during the day, she is prohibited to be intimate that entire day; if at night, she is prohibited that entire night. She always starts counting her niddah days and zivah days from this set time.
Hilchos Issurei Biah 8:4
Women must be careful regarding their designated times until they have definitely established the day and the hour when a period is expected. If her pattern was to begin menstruation on the 20th of the month and the 20th passed without her period arriving until the 23rd, she is prohibited to engage in intimacy on both the 20th and 23rd. If on a second occasion her period didn’t arrive on the 20th and it began on the 23rd, both days remain prohibited. If this happens a third time, the 20th is purified and her designated time is established as the 23rd. This is because a woman doesn’t establish a set time until it happens three three times, and a set time isn’t canceled until it doesn’t happen three times.