A woman who is a Nidda must observe a seven-day period called "Shib’a Nekiyim" – "seven clean days" – before she can immerse in a Mikveh to become permissible to her husband. The period of Shib’a Nekiyim cannot begin until the woman ascertains the complete cessation of bleeding, through an inspection called the "Hefsek Tahara." Shortly before sunset on the fifth day since she became a Nidda, she takes a special cotton cloth – which must meet certain specifications and can be purchased in any Mikveh – and thoroughly inspects the area to check for any residual blood. Some have the custom of washing the area before performing the inspection. If the cloth was clean after the inspection, without red stains, then the woman performs a second inspection, called "Moch Dahuk." She inserts a cloth into the area before sundown, and leaves it there until approximately thirty-five minutes after sundown. At that point, she removes the cloth and checks it for stains. If both these inspections did not indicate the presence of blood, then the woman is presumed to have stopped bleeding. She may then begin counting the Shib’a Nekiyim, and immerse in the Mikveh a week later.
For example, if a woman experienced the onset of menstruation on Sunday, then on Thursday, the fifth day, she would perform the Hefsek Tahara and Moch Dahuk inspections toward the end of the day, before sundown. If, for argument’s sake, sundown was at 6 pm, then she would perform the Hefsek Tahara inspection at some point before then, and if the cloth was clean, she would then insert the cloth for the Moch Dahuk inspection, shortly before 6 pm. She would remove and inspect the cloth at approximately 6:35-6:40 pm. If both the Hefsek Tahara cloth and the Moch Dahuk cloths were clean, then she begins the Shib’a Nekiyim that night, Thursday night. Assuming she does not spot any blood during the Shib’a Nekiyim, she immerses in the Mikveh the following Thursday night.
If the woman spotted blood on the cloth when she performed the Hefsek Tahara inspection, she may perform another inspection. In fact, she may repeat the inspection as many times as necessary so that she can succeed in making a clean inspection before sundown. Regardless of how many attempts are needed, she may begin her Shib’a Nekiyim if she performed a successful inspection before sundown, followed by a successful Moch Dahuk inspection. If, however, she was unable to perform a clean inspection before sundown, or if she found blood on the Moch Dahuk, then she cannot begin the Shib’a Nekiyim that night. She would instead have to try again the following day.
Summary: A woman who became a Nidda must observe a seven-day period of "clean days" before immersing in the Mikveh. This period begins after she has ascertained the cessation of bleeding. Five days after she becomes a Nidda, she performs an inspection with a cloth before sundown, and then, if the cloth is clean, she inserts a second cloth and leaves it there until approximately thirty-five minutes after sundown. If that cloth is also clean, then she begins counting the seven "clean days" that night, and immerses one week later.
