'Fifteenth' in the Bible

On the fifteenth day he sent the people away. So they blessed the king and went home to their tents rejoicing and with joyful hearts for all the goodness that the Lord had done for His servant David and for His people Israel.

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 offered upon the altar. So did he in Bethel, sacrificing unto the calves that he had made: and he placed in Bethel the priests of the high places which he had made.

So he offered upon the altar which he had made in Bethel the fifteenth day of the eighth month, even in the month which he had devised of his own heart; and ordained a feast unto the children of Israel: and he offered upon the altar, and burnt incense.