Leviticus 21:6 "They shall not profane the name of their G‑d"

The Torah gives us specific instructions on how to purify ourselves once we are impure.

One of the purification processes requires that the individual immerse himself in a "Mikvah" and then wait until sunset in order to become pure.

This person is known as a "Tevul Yom" - one who must wait until the sun sets in order to be pure.

This Negative Mitzvah cautions the priest who has become impure and who is a Tevul Yom not to omit any of the steps of purification before he returns to the Beit HaMikdash.

After immersing in the Mikvah, the priest must wait until sunset before returning to the Beit HaMikdash.