The Torah states, "You shall separate Beneh Yisrael from their impurity" (Vayikra 15:31). This verse has been interpreted as referring to a requirement to separate from one’s wife during the "Ona Ha’semucha Le’veset" – the time-period when she is likely to become a Nidda. Although this Halacha is inferred from a verse, this inference is actually just an "Asmachta" (an allusion to a law enacted later), and the prohibition applies on the level of Dibreh Soferim (Rabbinic enactment), as opposed to Torah law.
This Halacha requires a couple to abstain from relations throughout the period when the onset of Nidda is anticipated. If the wife had previously experienced the onset of Nidda during the day, then the period of abstention is the entire day when menstruation is expected, from sunrise to sunset. This applies regardless of when during the day the last onset of Nidda occurred, whether it occurred in the morning or in the afternoon. And if the last onset of Nidda occurred at any point during the night, then the period of abstention is observed throughout the night when menstruation is anticipated, from sundown to sunrise.
If the wife did not experience menstruation throughout the period of abstention, relations nevertheless remain forbidden until she performs a Bedika (thorough inspection) to ensure that no bleeding has occurred. After she performs a Bedika and no blood is discovered, the husband and wife may resume marital relations.
The Halacha of "Ona Ha’semucha Le’veset" applies regardless of whether or not the wife has a "Veset Kabu’a" ("fixed interval"). For example, if a woman regularly experiences the onset of Nidda every 29th day during the daytime, then she and her husband must abstain from sunrise to sunset of the 29th day, and may then resume relations after sundown and after she performs a Bedika. If a woman does not follow a fixed interval, and the onset of Nidda sometimes occurs on the 28th day, for example, and sometimes on the 29th, 30th or 31st day, then she must observe a number of periods of abstention when there is reason to suspect that she will become Nidda. One of these periods is the thirtieth day since the previous onset of menstruation. As women generally experience Nidda within thirty days of the previous onset of Nidda, a woman who does not follow a regular cycle must abstain on the 30th day since the previous occurrence. If the previous onset of Nidda occurred during the daytime, then she and her husband must abstain during the daytime on the 30th day; if the previous period of Nidda began during the night, then they must abstain during the night thirty days later. This Halacha is called Ona Benonit.
The exception to this rule is a woman who never experiences the onset of Nidda before the thirty-first day of the previous onset. If a woman’s interval is always at least thirty-one days, then she does not have to observe the Ona Benonit, since she does not have to anticipate the onset of Nidda before the thirty-first day. This is the ruling of Hacham Ovadia Yosef.
It must be emphasized that these Halachot are especially complex, and therefore couples must ensure to keep a calendar and carefully chart the wife’s Nidda cycle. Otherwise, the couple will not know when they must abstain, and might then violate this prohibition of "Ona Ha’semucha Le’veset," or, even worse, they may violate the grave Torah prohibition of Nidda if they engage in relations without realizing that the wife had already become a Nidda. Furthermore, many couples are unaware of the requirement for the wife to perform a Bedika after the period of abstention. It is mistakenly assumed that if no bleeding occurred during the period of abstention, relations are permitted immediately after the conclusion of that period. Couples must be reminded that a Bedika is required before relations may be resumed after the period of abstention.
