'Fifteenth' in the Bible

Jeroboam also inaugurated a religious feast in the eighth month on the fifteenth day of the month, like the religious feast which was in Judah, and he offered [sacrifices] on the altar. Thus he did in Bethel, by sacrificing to the calves that he had made; and he placed in Bethel the priests of the high places which he had made.

He offered [sacrifices] on the altar which he had made in Bethel on the fifteenth day of the eighth month which his heart had devised. He inaugurated a religious feast for the {Israelites}, and he went up to the altar to offer incense.