Why have Christians stolen Midwinters Eve from 20th December and renamed it Christmas Eve on the 24th?

They couldn't make the Yule holiday go away. It was powerfully ingrained in European culture, and it just makes sense to have an indoor, social holiday in the middle of winter so that this season is one of joy rather than the depression that the weather itself and lack of sunlight brings, which is probably why nearly every culture has a winter holiday. Since the holiday wouldn't go away they crossed out "Yule" and wrote "Christmas" on Dec 25th on the Calendar.... and next thing you know manger scenes are getting set up on this date... which brings us to where we are today.