'Fifteenth' in the Bible

And Jeroboam ordained a feast in the eighth month, on the fifteenth day of the month, like the feast that was in Judah, and he offered upon 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.

And he offered upon the altar that he had made in Bethel, on the fifteenth day of the eighth month, in the month which he had devised of his own heart; and he made a feast for the children of Israel, and he offered upon the altar, burning incense.