31:20 Your raiment - Namely, your spoil and prey. All work - All which had contracted some ceremonial uncleanness either from the dead bodies which wore them, or the tents or houses where they were, in which such dead bodies lay, or from the touch of the Israelitish soldiers, who were legally defiled by the slaughters they made.
31:19-24 The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.
