Niddah 72 - Eleventh Day
After the seven days of niddah , a woman immerses herself in a mikveh, and the couple can resume normal marital relations. During the next eleven days, if she sees blood, she is not called a niddah, but a zavah . Now she only has to wait for one day, and immerse in the mikveh the next morning. However, if she sees blood on the following day, the two days now combine, and her immersion in nullified. Therefore, the correct behavior in the eleven days of the zivah period is to wait until the end of the following day, and immerse in the mikveh then.
Our ruling is concerned with the eleventh day itself, when, should the woman see blood on the day following, twelfth one, the two days would not combine, and her early immersion in the mikveh would not be nullified. In that particular case, the woman saw blood on the eleventh day, went to the mikveh at night, not waiting till daybreak, and then had relations with her husband. Beit Shammai nevertheless give the couple all the stringencies of the regular zavah: both make objects ritually impure, and both have to bring a sacrifice. Beit Shammai extend the laws of the previous ten days to the eleventh one. However, Beit Hillel consider them non-guilty from the Torah point of view, and tell them not to bring a sacrifice. The Sages, however, had declared them ritually impure, to prevent this situation.
We will see the different sides of the argument on the next, concluding page of the Talmud.
Art: Caspar David Friedrich - Woman on the Beach of Rugen