1 Kings 12:30-33

Then this thing became a sin, for the people went as far as Dan to be before one. He also made temples on high places and appointed priests from among all the people, who were not of the Levites. And Jeroboam appointed a feast on the fifteenth day of the eighth month like the feast that was in Judah, and he offered sacrifices on the altar. So he did in Bethel, sacrificing to the calves that he made. And he placed in Bethel the priests of the high places that he had made. He went up to the altar that he had made in Bethel on the fifteenth day in the eighth month, in the month that he had devised from his own heart. And he instituted a feast for the people of Israel and went up to the altar to make offerings.