John Gill Commentary Numbers 31:24

"And ye shall wash your clothes on the seventh day, and ye shall be clean; and afterward ye shall come into the camp." — Numbers 31:24 (ASV)

And you shall wash your clothes on the seventh day
As the leper did, (Leviticus 14:9) :

and you shall be clean, and afterward you shall come into the camp ;
into the camp of Israel, into the camp of the Levites, and into the camp of God, the tabernacle.