Charles Spurgeon Commentary 1 Kings 12:32-33

"And Jeroboam ordained a feast in the eighth month, on the fifteenth day of the month, like unto the feast that is in Judah, and he went up unto the altar; so did he in Beth-el, sacrificing unto the calves that he had made: and he placed in Beth-el the priests of the high places that he had made. And he went up unto the altar which he had made in Beth-el on the fifteenth day in the eighth month, even in the month which he had devised of his own heart: and he ordained a feast for the children of Israel, and went up unto the altar, to burn incense." — 1 Kings 12:32-33 (ASV)

And he offered upon the altar. So did he in Beth-el, sacrificing unto the calves that he had made: and he placed in Beth-el the priests of the high places which he had made. So he offered upon the altar which he had made in Beth-el the fifteenth day of the eighth month, even in the month which he had devised of his own heart;

It is a strong condemnation of anything in religion if it is devised by a man's own heart. We are to do what God bids us, as God bids us, when God bids us, and because God bids us; but that which is merely of our own free will, ordained and manufactured by ourselves, is practically the worship of ourselves, and not the worship of God.