If a man has an emission during the time of darkness after sundown and wishes to immerse, when should he time his immersion? Should he wait until the morning the following day or must he wait until the following sundown to immerse? If he immerses before sunup on the day he has had the emission; is the immersion valid?
The rules of waiting before immersing in the mikvah were said in the torah regarding the laws of ritual purity. The rule is that depending on how many times the person had the emissions, he could be tameh for either one day or seven. These laws however unfortunately don’t apply nowadays because we are all ritually impure because of tumas mes (coming in contact or have been in the same building as a deceased person, and because we don’t have the ash of the red heifer). G-d willing when moshiach comes we will have these halachos back again.
There is another idea that even nowadays that a person that had an emission should immerse in a mikvah for kabbalah reasons, however there is no specific time of the day or waiting period that the person has to wait before immersing.