There are halachot applicable specifically to the time a woman is expecting her period. This time is called the vest. In Biblical times, and until recently, most women would get their period at a set time. Their cycles were very consistent. The period would either come on the same day of the month every time, or at set intervals of a specific number of days. Therefore, our Sages gave the day of the period special halachot, because a woman’s period was almost certain to start on that day. These halachot are perishah – separation, and bedikah – internal examination to check if her period has indeed commenced.
A woman who has an irregular cycle, must assume that each period may be the start of a consistent pattern. Therefore, such a woman needs to treat a number of days as her vest, for each of those days may be the regular pattern that will start. Even though in our times we don’t see regular periods as commonly as in the times of our Sages, we still must assume that a pattern is developing.
There are three types of vestot which will be addressed that a woman with an irregular cycle keeps. Since most women do not have regular cycles, the following halachot are applicable to almost everyone. If a woman sees a pattern, meaning she sees blood on the same vest three times in a row, a rabbi should be consulted.
Vestot are only calculated from a real period. A ketem, even if the women had to keep the laws of niddah because of the ketem, has no bearing on vestot.
A vest lasts one onah. An onah is one unit of time, either a day or a night. Day and night are defined by netz – sunrise, and shkiah – sunset. A woman calculates her vestot by the day or night in which she had the most recent period start. If it is unclear if the period started by day or night, because the period started right at shkiah or netz, we assume the period started at the later time. When calculating a vest, we follow the Jewish calendar where the night precedes the day.
There are some who have a custom to add on an additional onah to the vestot. This is known as the Vest Ohr Zarua, named for the halachic authority who originally instituted this custom. This would mean that if a vest was by day, the couple would treat the night before as a vest in regards to perishah (separation).
Calculating vestot can be confusing. There are a myriad of apps and websites that can help you keep track of all the important dates and times. MikvahCalendar.com is a good site. (We are not affiliated in any way with MikvahCalendar.com. They charge a fee of about twenty-five dollars a year.)