A woman anticipates her menses on certain days based on her previous menstrual pattern. These may include:
Yom Hachodesh (the day of the month)
The Haflagah (the interval between periods)
Onah Beinonit (the thirtieth day)
Veset Kavua (a regular period established over at least three cycles)
Veset Haguf (specific physical symptoms)
Vesatot are based on the Hebrew calendar, so you need to have one available for calculating them. You can buy a calendar specifically designed for vesatot, or you can use any Hebrew calendar.
It is also important to keep a running list of previous vesatot so you will know whether you have established a veset kavua. If you like, you can download a printable chart for this purpose. To see this calendar filled out, click here.
You also need an effective reminder when a veset is approaching. Many women mark the relevant dates on the calendar they use for calculations, or in their diary or phone.
How to Keep a Calendar:
When you get your period, you should mark an X on the appropriate Hebrew date, and indicate whether you began menstruating during the day or at night. It's a good idea to do this as soon as possible so you don't forget.
- Yom hachodesh is easy: turn to the next month in the calendar, and mark off the same date.
- For the haflagah, you need to count the number of days from your previous period to your current one, and then count forward the same number of days. It doesn't matter how you do this as long as you are consistent.
- Onah beinonit is Day 30 of your cycle (the day you get your period is Day 1). This is always four weeks and one day from the start of your period. So if you got your period on a Wednesday, onah beinonit will fall on a Thursday four weeks later. Onah beinonit is always either the same as yom hachodesh (after a 29-day month), or one day earlier (after a 30-day month).
All your onot perishah will be observed during the same onah (day or night) as the onset of your previous period. So if your most recent period began at night, all three onot perishah will fall at night.
If you establish a veset kavua, you only need to calculate your veset kavua. If you later get your period on a different day, you will follow the rules of uprooting a veset kavua.
If you had a distinct premenstrual symptom, you should indicate this as well. For example, you might write "severe cramps before period, same onah" or "back ache, two onot before period." If you find that the same thing happens three times in a row, consult a rabbi about how to proceed. If you are using hormonal contraceptives, you should indicate the date and onah on which you took the last active pill, removed the patch, etc.
You can also use your veset calendar to record the day you did your hefsek taharah and the day you expect to go to mikveh, in order to ensure that you don't get mixed up.
A bride should start keeping a veset calendar before her wedding, so that she will be able to observe the appropriate onot perishah once she gets married.
There are various computer programs and websites designed to calculate vesatot. Use them if you find this helpful. Just remember that computers are not foolproof. Make sure you understand how the vesatot work and what customs you keep so you can tell if errors occur.