1 Kings 12:32-33

32Jeroboam inaugurated a festival on the fifteenth day of the eighth month, like the festival celebrated in Judah. On the altar in Bethel he offered sacrifices to the calves he had made. In Bethel he also appointed priests for the high places he had made.

33On the fifteenth day of the eighth month (a date he had arbitrarily chosen) Jeroboam offered sacrifices on the altar he had made in Bethel. He inaugurated a festival for the Israelites and went up to the altar to offer sacrifices.