Numbers 19:12
He shall purify himself with it on the third day, and on the seventh day he shall be clean: but if he purify not himself the third day, then the seventh day he shall not be clean. He shall purify himself with it on the third day, and on the seventh day he shall be clean: but if he purify not himself the third day, then the seventh day he shall not be clean.
The law deals with defilement taken on through touching a corpse, and it lays down a two-stage cleansing that must take place on the third day and then again on the seventh day. The third-day washing is not optional or able to be made up later, for neglect on the third day leaves the defiled one still unclean when the seventh day comes. That corpse-uncleanness also serves as a figure for the moral defilement spread by close association with a corrupting person. To keep such a one as Terry in the house brings that same kind of uncleanness upon the whole household.