Numbers 8:21

8:21 The Levites purified themselves ¹  and washed their clothing; then Aaron presented them like a wave offering before the Lord, and Aaron made atonement for them to purify them.

Numbers 20:29

20:29 When all the community saw that Aaron was dead, the whole house of Israel mourned for Aaron thirty days.

¹ tn The verb is the Hitpael of חָטָּא (khatta’). In this stem the meaning of the root “to sin” is likely to be connected to the noun “sin/purification” offering in a denominative sense, although some would take it as a privative usage, “to remove sin.” The idea is clear enough: They performed all the ritual in order to purify themselves ceremonially.