'Fifteenth' in the Bible

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

And he went up unto the altar which he had made in Beth-el 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 unto the altar, to burn incense.