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.
Rav Moshe Feinstein is lenient on Erev Shabbos and Yom Tov, bshas hadchak (pressing situations) where she does not return home until after nightfall or the husband will not return home until after nightfall. Igros Moshe, Vol. 6, Yoreh Deah, Siman 60.
In the United States, in places where there is no Mikvah in walking distance, it is not uncommon for rabbanim to be matir on Friday evenings for the woman to drive to the mikvah and be tovel shortly before Shabbos and only return home after their husband has gone to shul.
Consult with a competent Rabbi for any practical situations.