Then he brought me to the entrance of the north gate of the house of the LORD, and I saw women sitting there, mourning the god Tammuz.
And in spring, people would celebrate the return of Ishtar and Tammuz around the time of the Spring equinox. That is why the celebration of Easter is based on the timing of the equinox and not based on when Passover happens. Also, many things associated with Easter have very pagan roots. The eggs and bunnies were fertility symbols associated with Ishtar, and ham is traditionally eaten at Easter to remember the boar that had killed Tammuz. God does not like religious syncretism, the mixing of pagan stuff with His worship.
In Deuteronomy 12 He says:
The LORD your God will cut off before you the nations you are about to invade and dispossess. But when you have driven them out and settled in their land, 30 and after they have been destroyed before you, be careful not to be ensnared by inquiring about their gods, saying, How do these nations serve their gods? We will do the same. 31 You must not worship the LORD your God in their way, because in worshiping their gods, they do all kinds of detestable things the LORD hates.
Also, I forgot to mention, that is also where the observance of sunrise services on Easter came from. It was to memorialize the return of the sun god Tammuz from the underworld.