1,109. The Time of the Immersion

162:2 A woman is not permitted to immerse on the seventh day until after the stars have come out. It is not even permitted to immerse close to nightfall so that she arrives home after dark. If she was delayed and didn't go to the mikvah on the night after the seventh day so that she immerses on a later day, nevertheless she may not immerse by day. Here also, she should be strict and not immerse close to nightfall, arriving home after dark. Rather, she should still immerse in the actual night. Brides who immerse before the wedding may do so on the eighth day or later by day. In a compelling circumstance, she may even immerse by day on the seventh day, even in the morning so long as it's after daybreak. In such a case, however, the wedding should not be held until after the stars appear that night. If she immerses after the wedding, even though this is her first immersion for her husband, the law for her is the same as for any other woman.

162:3 If she was affected by circumstances beyond her control, such as if she was afraid to immerse at night because of the cold or for safety reasons, or if the mikvah is out of town and her city locks the gates at night, then she may immerse on the eighth day during the daytime. However, she may not immerse on the seventh day in the daytime even if there are such circumstances. Since she is allowed to immerse on the eighth in the day due to these scenarios, she should do her preparations close to her immersion. If the eighth day is Shabbos or yom tov, she must do her preparations on the day before she immerses. Immersing is also not permitted on that day because we do not allow two leniencies on the same matter. The two leniencies in this case would be immersing during the day and distancing the preparations from the immersion.