Do you happen to have any ideas, or any sources, detailing why women first became required to bathe in the evening as in current practise, as opposed to the recorded biblical requirement to wash garments and bedding by day and to be declared clean in the evening? I am not referring to Niddah uncleanliness specifically, but am highlighting any uncleaness which a woman may come into contact from daily life including the uncleaness of men or women from an abnormal running discharge but in particular the uncleaness requiring daily bathing from the uncleaness which occurs from man and wife partaking in normal sexual relations.
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I apologize if this is confusing, but often there's some difference between what you'll get if you just open up and read a Bible, compared with the standard practices observed by Jews today.
Many of the Biblical laws of impurity aren't really of concern to us in our normal daily lives as we're not planning on entering the Temple, consuming sacrificial meat, or the like. Thus any time the Bible calls for "immerse in the day and pure at nightfall", well pretty much we don't worry about it today. We assume we're all ritually impure for now. And that doesn't affect too much at the moment. (Even in Biblical times, a person could be a very righteous and holy person, but employed as an undertaker and thus "ritually impure" almost all the time. They'd just have to take a break from their job and get "decontaminated" a few times a year to visit the Temple.)
The Second Temple was destroyed around the year 70 (i.e. about two thousand years ago); within about two centuries of that (we don't have an exact date, though I've seen some scholars trying to pinpoint it better), it's clear that people stopped trying to maintain ritual purity. Keep in mind that by that point:
The sage Rabbi Akiva (who died around 135, and had an expertise in the laws of ritual purity -- though it's unclear how practical those laws were at what point in his lifetime) stressed that theologically we need not worry about this too much: "Lucky you, people of Israel! Who purifies you? Your Heavenly Father!"
The law that has been as relevant then as it is now is that regardless of "ritual purity" status with regards to sacrifices and the like, there are the laws of Nida: when a woman starts menstrual bleeding, relations with her husband are prohibited; and they're not permitted again until bleeding stops, (then often a pause of clean time in between, that's a different question), then a night immersion whereupon, she can immediately resume relations. She could touch a corpse, dead lizard, every day throughout this entire month, that has no bearing on the laws of Nida.
I recently found the answer: Talmud Niddah 67b (soncino translation follows) in that women started bathing in the Evening following Rabbinic ruling that they should not bathe on the 8th day, lest their daughter see them and think they are bathing on the seventh day.