'Fifteenth' in the Bible

Jeroboam ordained a feast in the eighth month, on the fifteenth day of the month, like the feast that is in Judah, and he went up to the altar; so did he in Bethel, sacrificing to the calves that he had made: and he placed in Bethel the priests of the high places that he had made.

He went up to the altar which he had made in Bethel on the fifteenth day in the eighth month, even in the month which he had devised of his own heart: and he ordained a feast for the children of Israel, and went up to the altar, to burn incense.