Question: "Why did the time period of being unclean end in the evening?"
Answer: On numerous occasions in the Book of Leviticus, a person who had become unclean would only be unclean until evening. The answer to why is found in the Jewish understanding of how days operate. In Genesis 1, God’s week of creation is presented with each day having and evening and then a morning. The Jewish people recognized sunset or evening as the end of one day and the beginning of the next.
This is also why the Sabbath begins at sunset on Friday and lasts until sunset on Saturday in the Jewish tradition. The death of Jesus took place on a Friday, with His body buried before sunset of that day. No one visited the tomb on Saturday as it was the Jewish Sabbath and no work could be done on that day.
Under the Mosaic Law, several actions resulted in a person becoming ritually unclean until sunset. For example, Leviticus 11:24–25 states, “You will make yourselves unclean by these [unclean insects]; whoever touches their carcasses will be unclean till evening. Whoever picks up one of their carcasses must wash their clothes, and they will be unclean till evening.” If a person killed a fly and touched its carcass in the morning, say at 10:00 A.M., that person was to wash his clothes and would remain ritually unclean until that evening. At nightfall he would be considered clean again.
If this same man killed another fly and picked it up with his hands at 10:00 that night, he would again be considered unclean. He would have to wash his clothes again and remain unclean until the next night, at sundown. In Western thought, both occurrences would be seen as taking place on the same day (10:00 A.M. and 10:00 P.M.), but the Hebrew way of thinking considered these events as occurring on two different days. The result was the fly-swatting person being unclean for two days.
Imagine how often people would have to wash clothes—and in a time when the only possible way to wash was by hand (in a desert)! The Israelites would have taken the commands regarding uncleanness very seriously as a result.
An “unclean” person could not bring offerings before the Lord, nor could he participate in the festivals. For minor infractions, such as touching a bug, the “uncleanness” only lasted for the rest of that same day, ending at sunset.