Do not rejoice: This likely refers to the Feast of Tabernacles, when Israel celebrated the year's final harvest. God ordained this festival (Deuteronomy 16:13–15), but the Israelites turned it into a wild pagan celebration, acting like other nations.
you have played the harlot (literally you have loved a prostitute’s pay): The Israelites mistakenly believed their grain and wine were rewards for worshiping the Canaanite fertility god.