Leviticus 23:32

It shall be unto you a Sabbath of rest, and ye shall afflict your souls. In the ninth day of the month at even, from even unto even, shall ye celebrate your Sabbath. The exact length of the fast is specified, and the great rigor is to be noted with which the penalty of death was held forth for every transgression against the rest of the Sabbath and against the fast. The children of Israel mere to be made conscious, at least to some extent, of the heinousness and of the guilt of sin, that they might enter upon the celebration of the Day of Atonement with hearts full of genuine repentance.