1,077. Twelve Hours Prior

155:1 The Sages warned the Jewish people about uncleanness. Rabbi Yoshiya said that the Jews were warned to separate from their wives close to expected starts of their periods. How much time, precisely? Rabbah said 12 hours - either a day or a night. If her period is expected to start in the day - even if she normally sees blood at the end of the day - they must separate from the beginning of the day. Similarly, if she normally sees blood in the morning and didn't see it, she is still forbidden to have relations the entire day until nighttime. Similarly, if her period normally starts at night, even though she typically sees blood at the start of the night, or if she sees it at the end of the night, in any event she is forbidden to have relations the entire night. Many authorities say that not only marital relations are prohibited but also other forms of close contact. It would be appropriate to act stringently in this matter.

155:2 Some authorities say that the 12 hours when a man must separate from his wife are the 12 hours preceding the 12 hours in which her period is expected to begin. For example, if her period is expected at night, she is also forbidden to have relations the entire day before. If her period is expected during the day, she is also forbidden to have relations the entire night before. This is the proper way to behave. However, when the husband is departing on a journey, or returning from a journey, or if her mikvah night fell on the night before her period is expected, then one should not act stringently.