During the "seven clean days" that a woman must observe before immersing in a Mikveh, she must perform two daily inspections. The purpose of the inspections is to ascertain that she is indeed "clean" and the bleeding has completely stopped. She performs the first inspection in the morning after she wakes up, and the second inspection is performed before sundown. "Be’di’abad" ("after the fact"), it suffices if she performed one inspection during the course of the day. And even if she went through an entire day of the "seven clean days" without performing an inspection, she may still immerse as scheduled on the night after the seventh day if she performed at least one inspection on the first of the seven days, and one on the seventh day. If, however, she did not perform an inspection at all on the first day, she cannot begin counting until the following day, and if she does not perform an inspection on the seventh day, then she cannot immerse in a Mikveh that night.
There is, however, one exception to this rule. Maran (author of the Shulhan Aruch), in his Bet Yosef, notes a custom recorded by the Rashba (Rabbi Shlomo Ben Aderet of Barcelona, 1235-1310) that a woman performs a special inspection called "Moch Dahuk" before she begins the "seven clean days." On the afternoon before she begins this seven-day period, she inserts a cloth and leaves it there for approximately a half-hour after sundown, at which point she removes it to see if there are any bloodstains. The Bet Yosef writes that it is proper to observe this custom. If a woman performed a "Moch Dahuk" inspection before the "seven clean days," and she neglected, for whatever reason, to perform an inspection on the first day, then she may consider the "Moch Dahuk" her first day’s inspection. She may thus continue counting as usual, even though she did not perform an inspection on the first day.
Summary: During the "seven clean days" before a woman immerses in a Mikveh, she should perform two inspections each day to ensure the cessation of bleeding. If she did not perform two inspections each day, she may nevertheless immerse as scheduled after the "seven clean days" as long as she performed at least one inspection on the first day, and one on the seventh. If she did not perform an inspection on the first day, she may nevertheless continue counting as usual if she had performed the "Moch Dahuk" inspection the previous evening.
