Most women with a regular period expect their menses within a given time range but not on a specific day. A few women are able to predict the time of their menstruation exactly. Halachically, a woman has a veset kavua, an “established period,” if her menses follow a consistent and predictable pattern.
The most common forms of the veset kavua are the veset hachodesh, established when three consecutive periods begin on the same date of the Hebrew month, and the veset haflagah, established when four consecutive periods are separated by three equal intervals. The consecutive periods in question must all begin during the same onah (daytime or night-time).
There are other ways of establishing a veset kavua, but they are rare and rather complicated. If a woman notices any consistent pattern (e.g., menstruating on the same date every other month, or at gradually increasing or decreasing intervals), she should consult a rabbi or a yoetzet halacha.
A woman with a veset kavua observes only that veset. She abstains from relations and must perform an internal examination during the onah of the veset. If she forgot, she must do the examination as soon as possible afterwards, and may not engage in relations until she does so.
In contrast, a woman who does not have a veset kavua observes several onot perishah (times of separation): yom hachodesh, the haflagah, and onah beinonit. She abstains from relations and performs an internal examination during each of these onot. If she forgot to perform the examination on the onah beinonit, she must do so afterwards before she may resume relations; however, this is not required for the other onot perishah.
A woman with a veset kavua anticipates bleeding only on the day of the month or after the intervals she has established. If, however, she has a menstrual flow on an unanticipated day, then the following month she observes yom hachodesh and the haflagah from the deviant sighting, as well as her veset kavua. She does not observe onah beinonit.
If on three consecutive occasions the date of the veset kavua passes with no bleeding, then the veset kavua is “uprooted” and she no longer anticipates it. If, however, she subsequently has a menstrual flow in accordance with her former pattern, the veset kavua is immediately reestablished – even if this happens only once, and even if years have passed since the veset kavua was uprooted. A veset kavua can only be permanently uprooted if a new veset kavua is established.
A veset kavua may be based on physical symptoms which accompany menstruation, e.g. noticeable cramps that occur at a consistent time before menses (see veset haguf). A veset kavua is most commonly found in association with hormonal treatments (e.g., contraceptive pills – see Vesatot and Hormonal Treatments).
