A woman generally anticipates her menses on the same date of the Hebrew month (yom hachodesh) as the date of the onset of her period a month before. Yom hachodesh is an onat perishah (period of separation), on which marital relations are forbidden. In addition, a bedikah (internal examination) is performed on this day to check whether menstruation has begun.
Yom hachodesh lasts for one onah, either daytime (sunrise to sunset) or night-time (sunset to sunrise), corresponding to the onset of the previous menses. Thus, a woman who began to menstruate during the day on the 15th of Nisan will observe yom hachodesh during the day on the 15th of Iyar.
If a woman did not begin to menstruate on yom hachodesh, she does not anticipate her menses the following month on that date. Thus, if the woman described above began to menstruate only on the night of the 16th of Iyar rather than on the 15th, she would observe the night of the 16th of Sivan as the following month's yom hachodesh. Similarly, if she began to menstruate earlier, on the 13th of Iyar, she would observe the 13th of Sivan rather than the 15th.
If a woman began menstruating on the 30th day of the Hebrew month (the first day of a two-day Rosh Chodesh), and there are only 29 days in the next month, there is a dispute among the authorities. Most maintain that she should observe yom hachodesh on the first day of the following month (which is also Rosh Chodesh) because that is in reality the 30th day, but some hold that she need not observe yom hachodesh at all for that month, because there is no equivalent calendar date. A rabbi should be consulted.
A woman who begins her menses three consecutive times on the same date of the Hebrew month, and during the same onah, establishes a veset kavua (established cycle) for that date.