Talmudic Requirements
“And you shall warn the Jews to guard against their impurity.”
Based on this verse, the Talmudic sages established a requirement that the husband and wife separate on the date(s) when the next menstrual period can be expected to appear. This legislation was intended to prevent any opportunity for conjugal intimacy at the time of the anticipated appearance of menstruation.
Method: A record is kept of the onset of menstrual bleeding on an ongoing monthly basis. From this, the date of the next anticipated menstrual period is projected and intimacy suspended on that date.
Note: online purity calendars automatically project separation dates.
This includes marital relations, kissing, hugging, affectionate touching and sleeping in the same bed. The other restrictions observed during actual menstruation do not apply.
Three Dates of Separation
Numerous influences may affect the onset of menstruation. The Talmudic sages specified three cycles as most influential. These must be recorded and all three observed: the Monthly Cycle, the Average Cycle (30 /31 days) and the Interval Cycle. If some other regular pattern is noticed the couple should consult an orthodox Rabbi expert in these laws.
The Monthly Cycle Yom Hachodesh
The lunar cycle affects menstruation by influencing it to begin each time on the same date of the Hebrew (lunar) month. This is called the Monthly Cycle.
The Average Cycle Onah Beinonis
Another factor is that on the average for most women, the menstrual period commences on a regular thirty day cycle. This is called the Average Cycle.
The custom in many communities is to separate on the entire thirty first day, as well.
The Interval Cycle Veses Haflagah
This third possibility is the time interval from one sighting of menstruation until the beginning of the next. To calculate the Interval Cycle the wife must have completed one full cycle.
Separation for How Long?
In both the Monthly and Interval Cycles separation is observed for a half day period. A half day is considered to be from either sunset to sunrise or sunrise to sunset of the projected date. Either during the night half or day half of the date on which they occur.
In the Average Cycle separation is required to last a full day. (The Hebrew day begins at one sunset and ends at the next).
Or Zaruah: The custom in many communities is to separate an additional daytime or nighttime prior to the Monthly, Interval or Average Cycles above.
Examination
Additionally, on these dates, the sages required the wife to perform an internal examination to see if menstruation did, in fact, appear. This is accomplished by wrapping an examination cloth (or any soft white cloth) around her fingers, inserting it as deeply as possible, and turning it around while checking all crevices and folds. It is then removed and visually inspected for any sign of blood. (If discomfort is experienced the cloth may be moistened with water).
The examination is performed from the recorded time of day when the last menstruation began and onwards. For example, if the last menstruation began at 10:00 a.m. this examination should be performed anytime from 10:00 a.m. until the end of the day. Or, for example, if the last menstruation began at 10:00 p.m. this examination should be performed anytime from 10:00 p.m. until sunrise.
If there is blood, the wife enters the state of niddah. There is no minimum amount, even a spot no larger than a needle point renders her niddah.
If no blood is discovered, the couple is permitted to renew marital relations, providing that the day or night span (or complete night and day span in the Average Cycle) have passed completely. Regarding the Average Cycle, the wife must explicitly state to the husband that she checked herself.
If the wife misses the examination and no blood was noticed until the end of the Monthly or Interval Cycle then relations are permitted provided that the half day span has completely passed. Regarding the Average Cycle however, relations are forbidden until she does actually perform an examination.
Remember, for all purposes mentioned, nighttime is from sunset till sunrise. Daytime is from sunrise till sunset.
Beginning of the Cycle
Only the dates of actual menstruation or blood discovered through an internal examination, are recorded for the purpose of determining the anticipated period. Therefore, although stains found on the body or on clothes may render a woman niddah, nevertheless they are not considered when projecting the dates of the next menstrual cycle.