A Closer Look
Alternating 7-day menstrual periods and 11-day non-menstrual time spans: Inasmuch as—in this context—there is no empirical difference between menstrual and non-menstrual blood (the sole factor in determining whether blood is menstrual or non-menstrual being the day on which the bleeding occurs), it is quite easy to lose track of the count and mistake one type of blood for the other. Furthermore, as we shall see, the Torah forbids marital relations when the wife has been ritually defiled by either menstrual or non-menstrual bleeding, so the laws of this type of ritual defilement are just as pertinent when the Temple is not standing as when it is.
For this reason, in the second century, Rabbi Yehudah the Prince (the redactor of the Mishnah), in the wake of the upheavals accompanying the destruction of the Second Temple and the resultant diaspora of the Jewish people, decreed that whenever a woman bleeds for either one day or two consecutive days, she should count six clear days before immersing herself, and whenever she bleeds for three or more consecutive days, she should count seven clear days before immersing herself. This way, there is no need to keep track of the 7- and 11-day spans. If she bleeds for one or two days, she is technically permitted to engage in marital relations either after the second or third day after she bled (if it was one or two days of non-menstrual bleeding) or after the seventh day after she began to bleed (if it was one or two days of menstrual bleeding); thus, counting six clear days covers both possibilities. If, however, she bled for three or more days, three days of this bleeding might have been non-menstrual; she must therefore count seven clear days to cover that possibility.
However, the Jewish women of Rabbi Yehudah the Prince’s time took upon themselves to count seven clear days after any duration of bleeding, even only one day, in order that there be only one rule for all cases. This additional stringency was approved of by the legal authorities of the time and became fixed as Jewish law to this day.
