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

[A tevul yom is an individual who was impure and immersed himself in a mikvah (ritual pool). Though he is now pure, there are still certain restrictions upon him until the sun sets that day.]

A priest who is a tevul yom, though pure, is forbidden from doing any service in the Holy Temple until the sun sets.