'Fifteenth' in the Bible

Jeroboam invented a festival for the fifteenth day of the eighth month similar to the festival that takes place in Judah. He approached the altar that he had set up in Bethel and sacrificed to the calves that he had made, having stationed in Bethel the priests that he had appointed.

Then, on the fifteenth day of the eighth month, he went up to burn incense on the altar that he had set up in Bethel, thus beginning the festival that he had made up out of his own heart for the Israelis.