If a woman does not have a veset kavua, the couple will observe the following three onot perishah each cycle. They are calculated based on the day the woman’s last period began:
1) Yom hachodesh, the date of the Hebrew month when her period began.
For example, if a woman’s period began on the 4th of Tishrei during the day, then her yom hachodesh will be on the 4th of Cheshvan, during the daytime onah.
2) Veset Haflagah, the interval onah, calculated based on the number of days between the beginnings of a woman’s last two periods
For example, if 28 days passed between the beginnings of a woman’s last two periods, her veset haflagah will be on the 28th day after the start of the second period. It is observed on either that day or night, based on when her most recent period began.
(Couples who follow Chabad custom calculate the veset haflagah differently, see here.)
3) Onah beinonit, the thirtieth day from the onset of her last period.
If, for example, a woman’s period began on Sunday during the day, the onah beinonit will be on day 30, which is just over four weeks later, on Monday, in the daytime onah.
We follow the view that a woman with consistently long cycles (the last three intervals were 32 days or more) is exempt from observing yom hachodesh (which always falls out on day 30 or 31) and onah beinonit, unless she reverts to a shorter cycle.
