Veset Hayamim (Dates of anticipated menses)
A veset based on calendar dates is called veset hayamim (as opposed to a veset haguf, which is based on physical sensations accompanying menstruation).
Most women observe three types of vestot hayamim, which are calculated from the onset of their previous menses:
1) The same date of the Hebrew month, or yom hachodesh. If, for example, a woman began menstruating on the 15th of Nisan, then she anticipates her next menses on the 15th of Iyar.
2) The interval day, or haflagah. A woman anticipates that she will menstruate after the same interval as that which separated her previous two periods. If, for example, her last menses began 27 days after the beginning of the previous menses, then she anticipates her next menses 27days after the beginning of the last menses.
3) The thirtieth day from the onset of her previous menses, called the onah beinonit, the average interval.
Somewhat different rules apply to a woman with a veset kavua, a completely regular period as defined by halacha.
Each veset lasts for one onah, either daytime or night-time, corresponding to the beginning of the immediately preceding menses. For example, a woman who began menstruating during the day, and whose next period began after a 27-day interval at night, will anticipate her menses after a further 27-day interval at night.
According to some customs, additional periods of separation are required (some such customs are cited in onot perishah, onah beinonit). A custom adopted by a community as a whole is binding upon the entire community. Where there is no local custom, a couple inclined to observe these stringencies should consider the matter carefully and discuss it with a rabbi, as it would be halachically difficult for them to revert to the more lenient custom should they later wish to do so.