When a woman gets her period, she becomes ritually impure. She is halachikally obligated to count seven days from the cessation of her period, and then immerse in a mikvah. Upon immersion, she becomes ritually pure, and may have sex with her husband. If there is a temple, then immersion would also enable her to enter the temple; entry is reserved for those in a state of ritual purity.
There are a number of ways to become ritually impure: For example, seminal emissions can invalidate men, and touching a dead body can invalidate both men and women. One approach is to see ritual impurity as a result of loss of life – as in contact with a corpse – or of potential life: When an unfertilized egg or semen leaves your body, something with the potential for human life is no longer a part of you. Because most forms of impurity affect only one’s ability to partake of temple services, and we no longer have a temple, they’ve fallen by the wayside. Since a woman’s blood-induced ritual impurity affects her ability to have sex, it remains relevant.
In order to become pure the woman must count seven “days”, which are constituted as the 24-hour period between one night fall and the next. At the end of the seven days, she must immerse in a mikvah. Because a “day” is from one nightfall to the next, mikvahs are only open at night. (The preparation rooms might be open slightly before then in order to enable women to immerse precisely at nightfall.)
Having the mikvah open during such limited hours makes going to the mikvah extremely inconvenient for many women, and can also cause long lines. Especially for working women and/or women with children, night time can be very hectic, whereas perhaps ducking away for a longer lunch break or skipping out of work an hour early might be easier. The post-mikvah sex is traditionally seen as a sacred reunification between husband and wife, a time to celebrate their relationship. This special encounter can set the tone for the upcoming month, making it unfair to put women in a situation where the lead-up to that moment is hectic and stressful due to mikvah time constraints.
The logic is, that if the mikvah is open during the day, a woman might be so eager for sex with her husband, that she’ll come in during the seventh day, before nightfall, and immerse. Thus, she’ll have sex without having waited the full seven days, i.e. until nightfall. This logic demonstrates a lack of trust in women. Not only is that lack insulting, but it also threatens the entire edifice of mikvah, which is based on a woman’s trust-worthiness: The mikvah attendant can’t ensure that a woman did not have sex with her husband before immersing, or that she isn’t coming to immerse on the sixth night. If you don’t trust women, then short of putting police in everyone’s bedroom, there is no way to ensure that the laws are being followed, even by women who immerse at night.
Another worry is that you might see a woman going to the mikvah during the day and think she’s going too early, but that worry only applies if you either don’t trust the woman’s observance, or think that women can’t count. As for fears of other women emulating the daytime immerser, and through that, coming to immerse too early, there’s a very simple solution: Properly educate women.
Having the mikvah open during the day is based on the assumption that the woman in question has counted all her obligatory nightfalls, and chosen to immerse during the next day. Halacha generally discourages a time-lapse between the final nightfall and immersion, out of fears that pre-immersion sex may ensue. My argument against this concern is the same: Either we trust women, or we don’t. If we don’t trust them, having them immerse at night is futile, because they could still be breaking the laws in a myriad of ways. If we do trust them, we have to trust that they can go an extra night or two without breaking the rules.
There is a rabbinic precedent: The Rambam, in Hilchot Biah, 4:8, allows women to immerse on the eighth day under extenuating circumstances, such as sickness, the mikvah’s being far away, or fear of nighttime crime. His opinion follows the Talmud in Niddah, 67:2, which brings various cases in which women are allowed to immerse during the day.
It might be logistically difficult to have mikvahs open all the time, but surely it would not be difficult for mikvahs to have some daytime hours – say, to be open during mornings twice a week, or to be open from the end of the workday and onwards, regardless of what time nightfall is. Perhaps cities with many mikvahs might designate one as the daytime mikvah. There has recently been a lot of discussion about male oversight of the mikvah process; in fact however, the mikvah experience can be one of female empowerment: The mikvah grants spiritual significance to the natural processes of the woman’s body. It is up to the woman to count seven days, giving her control of her sexual and spiritual calendar. It is fellow women who pronounce the word “kosher”, signifying the woman’s renewed state of purity. While in the waters, the woman has the chance to turn her body into a vehicle through with she connects to the Divine. It is precisely because of mikvah’s potential as an experience of female empowerment, that the male control of the mikvah’s schedule, based on an underlying mistrust of women and their ability to control their sexual urges is so disheartening. If God asks women to exercise self-control for part of each month, surely He has faith that they can do so – so why don’t the rabbis?