Niddah 9:8

If a woman has a fixed time for her period, that is sufficient. The signs of a fixed time are that the woman yawns, sneezes, experiences pains in her stomach or abdomen, experiences a discharge, shivers or experiences some other kind of symptom before her period. If this happens three times, it is established as a fixed time.

Niddah 9:9

If the woman’s period typically began with the onset of her symptom, then anything ritually clean that she handled during the course of the symptom is ritually unclean; if at the end of her symptom, then anything ritually clean that she handled during the course of the symptom remains clean. Rabbi Yosi says that fixed periods can be particular days and hours. For example, if her period typically begins at sunrise, she is only prohibited to her husband at sunrise (because she’s expecting her period but she’s permitted the night before). Rabbi Yehuda says she has the whole day (if she didn’t get her period when expected).