Jeroboam thought to himself, "Unless I am careful, the kingdom will return to the dynasty of David.
When they go to Jerusalem to offer sacrifices at the Temple of the LORD, they will again give their allegiance to King Rehoboam of Judah. They will kill me and make him their king instead."
So on the advice of his counselors, the king made two gold calves. He said to the people, "It is too much trouble for you to worship in Jerusalem. O Israel, these are the gods who brought you out of Egypt!"
He placed these calf idols at the southern and northern ends of Israel -- in Bethel and in Dan.
This became a great sin, for the people worshiped them, traveling even as far as Dan.
Jeroboam built shrines at the pagan high places and ordained priests from the rank and file of the people -- those who were not from the priestly tribe of Levi.
Jeroboam also instituted a religious festival in Bethel, held on a day in midautumn, similar to the annual Festival of Shelters in Judah. There at Bethel he himself offered sacrifices to the calves he had made. And it was at Bethel that he appointed priests for the pagan shrines he had made.
References for 1 Kings 12:32
So on the appointed day in midautumn, a day that he himself had designated, Jeroboam offered sacrifices on the altar at Bethel. He instituted a religious festival for Israel, and he went up to the altar to burn incense.