Israel's king was Jeroboam I. Judah's king, from David's line, was Rehoboam. Also, with Judah went the bulk of the Levites.
Now, Jeroboam was a politically astute man. I believe the Bible calls him "clever." So he was quick to recognize the possibility that he would lose his kingdom, lose his following, if the people continued to travel south to Jerusalem to keep the Feast. Thus, he made a politically astute move by promoting another religion. Even though it says that "he said in his heart," by no stretch of the imagination does this mean that he invented a "new" religion. What he did was incorporate into his system, and give governmental backing to, already ongoing pagan practices.
But right now we are talking about the fifteenth day of the eighth month, in particular, and the fifteenth day of any month, in general.Remember that we are still, here, talking about Israel (the northern ten tribes) and a very astute political move made by their first king, Jeroboam I—by instituting already ongoing pagan practices as a part of the official governmental policy instituted under him.
What Jeroboam was doing was not just setting up a rival kingdom against Judah, but rather a full-fledged Babylonian religious and governmental system. Once he made the jump into the religious area, it was now set to become part of everyday life. So he had built a new temple, altar, priesthood (with himself as high priest); and, on top of that, he was king. It was dedicated on the very day that the pagan kings designated as a day of homage and sacrifice to their god-kings.
