Midrasz do Kapłańska 22:4

1) (Vayikra 22:4) ("A man, a man, of the seed of Aaron the Cohein, if he is a leper or a zav (see section 15), of the holy things he shall not eat until he is clean. And one who touches anyone who is unclean through the dead; or a man from whom semen issues,") "the seed of Aaron": This tells me only of the seed of Aaron. Whence is Aaron himself derived (as subsumed in the prohibition)? From "if he is a leper or a zav." "of the holy things (terumah) he shall not eat until he is clean": (i.e., when the sun goes down,) but Israelites may eat ma'aser t'vulei yom (having immersed themselves in the daytime). Whence is it derived that Aaron and his sons (may eat ma'aser, t'vulei yom)? It follows a fortiori, viz.: If Israelites, who may not eat of terumah (even) when the sun goes down, may eat of ma'aser t'vulei yom, then Aaron and his sons, who may eat of terumah when the sun goes down — how much more so may they eat of ma'aser t'vulei yom! Israelites, then, are derived (by implication) from the verse, and Aaron and his sons, a fortiori (as being permitted to eat ma'aser t'vulei yom.

1) Above (Vayikra 23:2), intercalation of the year is being spoken of, and here (Vayikra 22:4) sanctification of the month is being spoken of. If (the moon) were seen clearly, or witnesses came and testified (to that effect) before them (beth-din), and they were unable to proclaim "It is intercalated until it became dark — whence is it derived that it is intercalated? From "which you shall call (them) in their times." If you called them, they are My festivals. If not, they are not My festivals.

2) "until he is clean": I might think, until he immerses; it is, therefore, written (Vayikra 22:7) "And when the sun sets, he shall be clean." Just as his being clean below is with the setting of the sun, so, his being clean here.