When a woman becomes a Nidda, she must wait five days before beginning to count the seven "clean days," at the completion of which she may immerse and then become permissible for her husband. Even if she can confirm the cessation of bleeding after just a day or two, she may not begin the seven "clean days" until after five days from the onset of bleeding. The reason for this Halacha is that after a woman engages in relations with her husband, she is in a state of impurity for several days due to the impure substance in her body. In order to ensure that there is no longer any Tum’a (impurity) inside her, she must wait five days and only then begin the period of the "clean days."
In principle, this line of reasoning applies only if a woman was with her husband just before the onset of bleeding. But if relations had not taken place for several days prior, then, seemingly, she should be permitted to begin the seven "clean days" as soon as she confirms the cessation of bleeding, even if this occurs after just a day or two. Nevertheless, the Rama (Rav Moshe Isserles of Cracow, 1525-1572) writes that the Sages enacted a provision requiring waiting five days regardless of whether relations had taken place. This is, indeed, the custom in our community, and thus even if the woman knows that she was not with her husband during the week before the onset of bleeding, she must wait five days before counting the seven "clean days."
There are, however, a number of exceptions to this rule. One such case is where a woman experiences bleeding after she had already waited five days and begun the seven "clean days." In this case, since she already waited five days, the woman may begin a new period of seven "clean days" immediately when she ascertains the cessation of bleeding. Another exception is where a woman experienced bleeding after immersing in the Mikveh, before engaging in relations with her husband. In this case, too, she may begin the seven "clean days" immediately upon determining the cessation of bleeding, without first waiting five days. Although the Noda Bi’yehuda (Rav Yechezkel Landau of Prague, 1713-1793) ruled that the woman must wait five days in this case before beginning the "clean days," many other authorities (including the Hatam Sofer, the Peneh Yehoshua, Rav Abdullah Somech, and the Ben Ish Hai) disagreed, and this is the accepted view. Thus, as Hacham Ovadia Yosef rules in Taharat Ha’bayit (vol. 2, p. 419; listen to audio recording for precise citation), if a woman experiences bleeding after immersion before engaging in relations with her husband, she may begin the seven "clean days" as soon as she can determine the cessation of bleeding, without having to wait five days.
Summary: Once a woman begins bleeding, she must wait five days before she can begin the seven "clean days" after which she may immerse in a Mikveh. Even if she can establish the cessation of bleeding before five days, she does not begin the seven "clean days" until after five days. The exception to this rule is where a woman experiences bleeding during the seven "clean days," or if she experiences bleeding after immersing in a Mikveh, before engaging in relations with her husband. In both these situations, she may begin the seven "clean days" as soon as she ascertains the cessation of bleeding, even without waiting five days.
