Onot Perishah (times of separation)
A woman anticipates her menses on certain veset days, based on her previous menstrual pattern. Marital relations are forbidden on these days in order to minimize the risk that menstruation could begin during intercourse, causing an inadvertent violation of the laws of niddah. These days of separation are called onot perishah. Most women observe onot perishah on yom hachodesh, the haflagah, and onah beinonit.
Sexual intercourse is forbidden during the onat perishah. Other forms of affection are permitted; you and your husband should use common sense in deciding what's appropriate. There is a custom in some communities to refrain from hugging and kissing. Additional restrictions which apply when the wife is a niddah (harchakot) do not apply at these times.
During the onat perishah, the woman must check whether she has begun to menstruate as anticipated. Because she becomes niddah as soon as blood exits the uterus into the vaginal canal, even before it leaves her body, she may not rely on an external inspection alone. Minimally, she must perform at least one internal examination (bedikah) at some point during the onat perishah. Some women do two bedikot (beginning and end) or three (beginning, middle and end), but if the examinations are painful, or if the irritation could itself lead to a problematic bedikah, then one is sufficient.
If a woman forgot to perform a bedikah on the onah beinonit (the thirtieth day from her previous menstrual period) or on her veset kavua (established period), she MUST do one afterwards and may not resume relations with her husband until she has done so. If, however, she missed the bedikah on another onat perishah, it is desirable but not required that she do one afterwards.
Each onat perishah lasts for one onah, either daytime or night-time, corresponding to the beginning of the previous menstrual period. For example, a woman who began menstruating on the 15th of Nisan at night will observe an onat perishah on the 15th of Iyar at night.
Some couples observe the onah prior to the veset as an additional onat perishah. This custom is known as the onah of the Or Zarua (named for the 13th century authority who argued for it). If, for example, a woman began menstruating on the 15th of Nisan at night, the regular onat perishah falls on the 15th of Iyar at night, and the additional onat perishah of the Or Zarua falls on the previous onah, the 14th of Iyar during the day. Hence, a couple observing this practice must refrain from relations during the 24-hour period, from sunrise on the 14th to sunrise on the 15th.
Definition of Onah
The word onah literally means "time period." In the context of the laws of niddah, it usually refers to a day or a night. Each 24-hour day thus consists of two onot. The daytime onah begins at sunrise (henetz hachamah, commonly called netz) and ends at sunset (shekiat hachamah or shekiah). The night-time onah lasts from sunset until sunrise.
Marital relations are forbidden on an onah (as defined above) when a woman anticipates her menses. This is called an onat perishah, "time period of separation."
The term onah can also refer to the length of the menstrual cycle. Halachically, we assume that the onah beinonit, or "average interval," is thirty days long.
Finally, the term mitzvat onah (a mitzvah performed at a set time period) refers to a husband's conjugal obligations toward his wife and is also used as a halachic euphemism for marital relations.