There are parts of CD Projekt Red's amazingly deep and challenging adventure The Witcher 3: Wild Hunt that are covered in snow already, but if you want the entire game world all white in time for Christmas, you need a mod and that mod is called "Winter is Coming", that was made by KNG and is free to download here.
Now, if someone could juar make Geralt fat, red, and with a big white beard, everything would be perfect in time for Christmas. Are there any other Christmas mods which get you in the festive spirit?