A woman who became a Nidda immerses in a Mikveh only after observing the period of Shiva Nekiyim seven days in which no blood was discovered. According to the custom of Halab, she may begin the period of Shiva Nekiyim only after the fifth day at least since the onset of menstruation.
Furthermore, in order to begin the Shiva Nekiyim, the woman must establish the cessation of the flow of blood by performing an inspection with a cloth, on the fifth day (at earliest) since the bleeding begun. If no blood is discovered at this inspection, then she may begin counting the seven days.
This inspection, which is called "Hefsek Tahara," should ideally be performed within a half-hour before sundown. If the woman was unable to perform the inspection before sundown, then she may still do so until 13.5 minutes after sundown. This is the view of Hacham Ovadia Yosef. However, one should consult a Rabbi in any case when the Hefsek was performed after sunset.
If the woman customarily recites Arbit, she should preferably perform the Hefsek Tahara before reciting Arbit. Even though Arbit may be recited before sundown, the Hefsek Tahara should be performed before the recitation of Arbit. Nevertheless, a woman who recited Arbit may still perform the Hefsek Tahara inspection, provided, of course, that the sun had not set (or at least it is within 13.5 minutes after sundown).
If a woman performs a Hefsek Tahara on Erev Shabbat, she should do so before lighting the Shabbat candles. Once again, however, if she mistakenly lit the Shabbat candles first, she may still perform the Hefsek Tahara so long as she remembers before the final time, as discussed above.
Summary: A woman who became a Nidda immerses in the Mikveh after observing the period of "seven clean days," which begin after at least five days have passed since the onset of menstruation. In order to begin the "clean days," she must perform the "Hefsek Tahara" inspection, which should be done within a half-hour of sundown, and no later than 13.5 minutes after sundown. Women who pray Arbit should preferably perform the Hefsek Tahara before Arbit, and on Erev Shabbat the Hefsek Tahara should, preferably, take place before candle lighting. (Note- Rav Moshe Feinsteins opinion is that, Bediavad, one has only up until 9 minutes after sundown to perform the check.)
