M'Dioraysa a zavah gedolah can go to the mikvah on the morning of Day 7 as Miktzas Hayom K'Kulo. However, the rabanan were gozer that a women must wait until nightfall to go. The concern is: If she were to go during the day, and come home and have relations with her husband during the day, and afterwards would see blood before nightfall, that would invalidate her Shivah Nikiyyim. My question is: Is it ever permitted for a women to go during day 7? I believe I once saw a heter to go on Erev a 3-day Yom Tov if otherwise mikvah would be pushed off 3 days. I have heard of heterim in cases where the couple wouldn't see each other until nightfall. I was unsuccessful at locating any sources.