It is very rare for a woman to be permitted to immerse on the seventh day during the day. In any case, individual circumstances vary and one cannot extrapolate from one situation to another.

The reason for not allowing immersion during the day on day seven is that, if a woman were to start bleeding later in the day, she would retroactively invalidate her immersion. We do not want the couple to be tempted by the fact that she has already immersed and have relations before nightfall, lest her immersion be invalidated retroactively. If a woman does immerse on the seventh day without a unique halachic permission, the immersion is usually treated as invalid and must be repeated after nightfall.

The prohibition on daytime immersion on the eighth day is a rabbinic decree, lest a woman’s daughter see her going during the eighth day and incorrectly conclude that daytime immersion is permitted on the seventh day. This decree applies to all women whether or not they have daughters (and is also applied to the seventh day in addition to the above). It does, however make provisions for some extenuating circumstances, such as the mikveh being in a location that is unsafe or inaccessible at night (when there is no other option available), and it allows for leniency to permit daytime immersion for brides.

When immersion on the eighth day is permitted, there is a debate as to whether the wife may see her husband prior to nightfall. According to all positions in this debate, marital relations are not encouraged during the day and should still wait until after nightfall, as they would on day seven. The reason woman may be advised not to see her husband until nightfall is to prevent the temptation of intimacy when she has already immersed and as a way to prevent her making erroneous analogies to an immersion on day seven.