The Gemara in Masechet Shavuot (18b; listen to audio recording for precise citation) establishes the requirement for a husband and wife to abstain from intimacy at certain times when the onset of the wife’s menstrual flow is anticipated. This Halacha is inferred from the verse in the Book of Vayikra (15:31), "Ve’hizartem Et Beneh Yisrael Mi’tum’atam" ("You shall separate the Israelites from their impurity"). The Gemara explained this verse to mean that one must separate from his wife on the occasions when she is prone to becoming impure. Even though no blood has been sighted, and, as such, the wife is still Tehora (ritually impure), the couple may not engage in intimacy, given the likelihood of the wife’s becoming Nidda during these periods.
When exactly must a husband and wife separate?
If a woman generally experiences the onset of her cycle at a fixed interval, for example, every twenty-eight days, then she must separate from her husband on the day her flow is expected based on this interval. If her previous flow began during the daytime, then they must separate during the daytime twenty-eight days later; if her previous flow began at night, then they must separate throughout the night twenty-eight nights later.
It is very common, however, for women to have irregular cycles, and to experience Nidda at a different interval each month – sometimes twenty-seven days after the previous flow, sometimes twenty-eight, etc. Women who experience the onset of Nidda at irregular intervals must observe three periods of separation (besides, of course, the period when she is actually a Nidda). First, she must separate from her husband at the time when the previous interval of time has passed. For example, if her previous flow began twenty-seven days after the preceding one, then she must observe a period of separation when twenty-seven days have passed since the previous flow – either by day or by night, depending on when the previous flow began. Secondly, she must observe the period of "Ona Benonit" thirty days after her previous sighting. Once thirty days have passed since the onset of her previous flow, then she must separate from her husband for one period (day or night), given the likelihood of her experiencing Nidda. Finally, she and her husband must refrain from intimacy when a calendar month has passed since the onset of her last flow. For example, if a woman became a Nidda on the seventeenth of Tebet, then she must separate from her husband one month later, on the seventeenth of Shebat, either by day or by night, depending on when her previous flow began.
It is therefore imperative for every married woman to keep a detailed record of when her menstrual flow begins each month. She must make note of how many days had passed since the previous flow began, of the calendar date, and of whether it occurred by day or by night, so that she can calculate the periods when she and her husband must refrain from intimacy.
What exactly is forbidden on these occasions?
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) rules that this prohibition applies only to marital intimacy, and not to the other activities that are forbidden when a woman is a Nidda. For example, a husband and wife may touch each other in a non-intimate fashion and hand objects to one another during these periods of separation. Intimate physical contact ("Hibuk" and "Nishuk"), however, is forbidden. The Ben Ish Hai also writes that it is proper for a husband and wife to sleep in separate beds during the periods of separation, given the likelihood that the wife will become a Nidda during the night. This is also the position of Hacham Ovadia Yosef. Furthermore, the Ben Ish Hai writes that a husband must carefully determine which activities may arouse his Yeser Hara and avoid these activities during the periods of separation.
Unfortunately, many people are unaware of these Halachot of "Vestot" ("cycles"). There are those who carefully observe the laws of separation during the period of Nidda but are not cognizant of this requirement to separate on the occasions when the onset of Nidda is expected. Since the wife is still Tehora, she and her husband and not in the mindset of requiring separation. It is important to emphasize that even though the woman is not a Nidda, intimacy is forbidden during these periods, given the likelihood of the woman becoming a Nidda at these times.
Summary: A husband and wife must refrain from intimacy at the time when the onset of the wife’s menstrual flow is anticipated. If the woman experiences her flow at irregular intervals, she and her husband must separate on three occasions each month (besides, of course, the period when the wife is a Nidda):
1) At the time when her flow can be expected based upon her previous interval.
2) On the day of the month when she experienced Nidda the previous month.
3) Thirty days after the onset of her previous flow.
During these periods, the husband and wife must avoid intimate physical contact, and should sleep in separate beds.
