A woman must wait at least five days from the time she becomes niddah until she can begin counting the seven blood-free days. She counts the day on which she became niddah as the first of these five days, and then counts four full days (from sunset to sunset, according to the Jewish calendar). If she has stopped bleeding, she may perform her hefsek taharah prior to sunset on the fifth day.
Thus, a woman who becomes niddah between sunset on Saturday and sunset on on Sunday may do her hefsek taharah on Thursday – the fifth day. She begins counting her seven blood-free days on Thursday night and Friday day (the sixth day from becoming niddah). Thursday is the earliest day for her hefsek taharah.
Some Sephardic authorities permit women to perform a hefsek taharah on the fourth day and begin the seven blood-free days on the fifth day (both a day earlier than what we presented above). In that case, in our example, a woman who has stopped bleeding by Wednesday could perform her hefsek taharah then.
Many women have longer menses and, in practice, are not able to perform a hefsek taharah as early as the fifth day of their period.
Even a woman whose menses are short, or who has become niddah because of mid-cycle staining, may not start her seven blood-free days until the sixth day from the onset of her niddah status.
However, a halachic authority may allow a wait of only four days when there are very extenuating circumstances, such as a couple having great difficulty conceiving.
When a bride has become niddah from dam betulim (hymenal bleeding), a four-day wait is sufficient according to all traditions. However, if she is also gets her period, she needs to wait the usual five days.
The minimum days begin when a woman becomes niddah. A woman who is already in the niddah status, and experiences a new episode of bleeding, does not need to count the minimum days again. For example, a woman who experiences bleeding during her seven blood-free days may perform a new hefsek taharah as soon as the bleeding stops. Similarly, if a woman never immersed in the mikveh following her menstrual period (e.g., she and her husband were in different cities for the entire month or she is a bride before her wedding), she does not need to wait the minimum days when she gets her next period.
The source of this halacha is as follows:
Women commonly expel semen for a few days after intercourse. A day on which this occurs cannot be counted as one of the seven blood-free days.
A woman who is unaware that she has expelled semen may mistakenly count the cancelled day and immerse in the mikveh too early. Because such an immersion would be invalid, she might unknowingly violate the laws of niddah.
From a halachic standpoint, semen is considered viable for 72 hours, after which it has no further halachic significance. Therefore, to be certain there can be no halachically significant seminal expulsion during the seven blood-free days, at least 72 hours must elapse from the time a woman has intercourse until she begins to count. Moreover, since the 72 hours may begin in the middle of one calendar day and extend to the middle of the fourth day, she cannot start to count the seven blood-free days until the fifth day.
That is why some Sephardic authorities permit women to perform a hefsek taharah on the fourth day (not the fifth). The halacha in most communities, however, is to wait until the fifth day, to avoid problems should a woman have intercourse shortly after sunset, and mistakenly count the outgoing day as the first of the four days.
Beginning the Minimum Wait
The five day minimum begins from the day a woman enters the status of niddah, even if the cause is not a flow. Therefore, if she becomes niddah from a stain on clothing or a bedikah cloth, or from a medical procedure, the day she becomes niddah is day one of the five day minimum.
The five day minimum also begins from the point in time at which a couple acts as if niddah has begun (including observing the harchakot) out of doubt concerning niddah status. For example, a woman discovers a questionable stain on Monday. She tells her husband that she might be niddah; the couple immediately begin to follow the laws of niddah out of doubt over her status and she sends the stain for halachic evaluation. On Tuesday, before she has received a ruling on the stain, she begins her menses and is definitely niddah. She may count Monday, the day she acted as niddah, towards the five day minimum even if it is later determined that the staining on Monday did not make her niddah.
However, when a woman has non-niddah staining and a couple abstains from relations as a precaution against a flow beginning during relations, this abstention does not count toward the five day minimum.
If the distinction between halachic prohibition and careful practice is unclear, an individual halachic question should be asked.
The minimum wait was instituted to avoid expulsion of viable semen during the shivah neki'im. However – for all Ashkenazim and in some Sephardi communities – it is required even where the couple did not actually have intercourse for some time before the onset of niddah. This ensures unity of practice and applies even if relations were completely impossible before the woman became niddah (e.g., the husband and wife were in different cities).
In some Sephardi communities, the minimum days are counted from the last time the couple had relations before niddah and in some cases, a woman can further shorten or even eliminate her wait by washing internally following relations.
Questions arise when relations are halachically forbidden for reasons other than niddah. Can those days be counted towards the minimum? There is some halachic debate in this area, so in practice an individual question should be asked. Here we explore several such situations one by one:
Marital relations are forbidden on veset days (onot perishah). If a woman has a full 24-hour veset immediately followed by another veset, and becomes niddah during the second veset, she may be able to count her minimum days from the beginning of the first veset. This situation can apply to those who keep onah beinonit for 24 hours and have another onat perishah come out the following evening. It can also apply to women who keep the onah of the Or Zarua prior to every veset, have two of these come out back to back, and start to bleed during the latter one. So too, days of separation due to a veset haguf may be counted towards the five day minimum.
Yom Kippur or Tisha B'Av
Marital relations are forbidden on Tisha B'Av and Yom Kippur. If a woman becomes niddah during a nighttime onat perishah immediately following one of these fast days – so that relations were prohibited continuously for the 24 hour fast day and the following night – some poskim allow the fast day to be considered the first day of the minimum wait.
Marital relations are forbidden when either the wife or husband is sitting shivah. If the wife becomes niddah during aveilut, some halachic authorities would allow the minimum days to begin from the onset of aveilut, while others would not.