'Fifteenth' in the Bible

Jeroboam held a feast on the fifteenth day of the eighth month, like the feast which is kept in Judah, and he went up to the altar; he did this 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.

So he went up to the altar which he had made in Bethel on the fifteenth day of the eighth month, in the month which he had devised in his own heart [in defiance of God’s commandments]; and he held a feast for the Israelites and he went up to the altar to burn incense [in defiance of God’s law.]