Eleazar the priest said to the men of war who had gone to a battle,
“And you shall wash your clothes on the seventh day and be clean, and afterward you may come into the camp” (Numbers 31.24).
After mixing and mingling with unclean people during the war, the Israelite men were then unclean, and so the Lord prescribed for them what they had to be clean again.
Otherwise, they had to stay outside the camp until they were clean.
Purification and cleanliness in the spirit is a top priority with the Lord, and thus on the seventh day, the Israelites could be perfect again by washing their clothes on the seventh day.