1 Kings 12:32-33

32Then Jeroboam instituted a festival in the eighth month, on the fifteenth day of the month, imitating the Festival that is in Judah. He went up to the altar that he built in Bethel, to sacrifice to the calves that he had made. He installed in Bethel the priests of the high places that he made.

33Then he went up to the altar which he had made in Bethel on the fifteenth day in the eighth month—in the month that he had made up from his own heart—and made up a festival for the children of Israel, and went up to the altar to burn incense.