A woman who became a Nidda must observe the "Shiba Nekiyim," or "seven clean days," as part of the process of removing the status of Nidda. She must perform inspections on each of the seven days to ensure the complete cessation of bleeding.
The inspection on the seventh day is "Le’ikuba," which means that it is indispensable for the process of removing the status of Nidda. A woman who did not perform an inspection on the seventh day cannot immerse in the Mikveh that night.
When is the final time on the seventh day for performing the inspection? If a woman did not perform the inspection before sundown, must she wait until the next day, or does she still have the opportunity to inspect herself that day?
Hacham Ovadia Yosef rules that although a woman should perform the inspection before sundown, a woman who did not perform the inspection by that point may still do so during the period of "Ben Ha’shemashot" ("twilight," the period between sundown and nightfall). This period extends for approximately 13.5 minutes after sundown. Therefore, if a woman failed to perform the required inspection on the seventh day before sundown, she may still do so, provided that she is still within 13.5 minutes after sunset, and she may then immerse in a Mikveh that night. If she did not perform the inspection before the end of this 13.5-minute period, then she has missed the opportunity to perform the inspection that day, and she must perform the inspection the following day, and then immerse that night.
Summary: A woman who became a Nidda must perform inspections each day of the seven-day period of "Shiba Nekiyim." If she did not perform an inspection on the seventh day, she cannot immerse in the Mikveh that night. If she did not perform the inspection before sundown on the seventh day, she may perform the inspection within the period of 13.5 minutes after sundown.
