Charles Ellicott Commentary Numbers 19:12

"the same shall purify himself therewith 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." — Numbers 19:12 (ASV)

He shall purify himself ... — The verse may be rendered as follows: He shall purify himself with it on the third day and on the seventh day; so shall he be clean; but if he does not purify himself on the third day and on the seventh day, then he shall not be clean: so the Septuagint and Vulgate .