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