Remember, the Israelites carried idols with them throughout their time in the wilderness (Amos 5:25-26; Acts 7:42-43). Their idols seemed good to them, but the true God did not. God's way of life undoubtedly seemed devilish - it was chaffing and constraining, which is one reason they always complained. They did not see the true God as a force for good in their lives. They believed He was bent on their destruction (Deuteronomy 1:27-28). Joshua rebuked Israel because, as he said, serving the true God seemed evil to them (Joshua 24:15)! The Israelites accepted Baal, Molech, and other demons, and yet, as Stephen told the council during his trial, the fathers always resisted the Holy Spirit (Acts 7:51).
That syncretistic festival also rhymes with what God says in Amos 5. He says, I hate, I despise your feast days, and I do not savor your sacred assemblies. . .. He says, Take away from Me the noise of your songs, for I will not hear the melody of your stringed instruments (Amos 5:21-23). A few verses later, God says that even as they were making sacrifices to Him in the wilderness, they also had other gods. And then He says, Therefore I will send you into captivity . . . (Amos 5:25-27). In another chilling rhyme, 40 of the revelers were taken into captivity on their feast day.