'Fifteenth' in the Bible

Jeroboam instituted a feast in the eighth month on the fifteenth day of the month, like the feast which is in Judah, and he went up to the altar; thus he did in Bethel, sacrificing to the calves which he had made. And he stationed in Bethel the priests of the high places which he had made.

Then 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 in his own heart; and he instituted a feast for the sons of Israel and went up to the altar to burn incense.