The return of Christmas mode!
Much better idea: Make a mod to do it. That way only the people who WANT to play such a thing have to, and the rest of us can play normally.
Christmas mode, along with the Easter Egg hunt, were system clock time based, and as such had a lot of issues. They were both removed from SRB2. The only remaining date-based thing in SRB2 is the AFD intro, which doesn't do anything to the gameplay.
Christmas mode in particular was subject to a lot of hatred in the SRB2 community, mostly due to the wonderous ability of it to manifest itself at horrible times and cause bugs everywhere. The nature of SRB2's netplay is that it expects everyone to have the same files loaded and the same content visible, but XMas mode was an exception to that. Even worse, XMas mode put precipitation into every map, causing massively inconsistent load times between players. XMas mode has been particularly notorious in the past for causing consistency failures because some players would have their system time set to the date range for it and some people wouldn't.
The bottom line is that such a concept, while cute, just isn't a good idea in a multiplayer game. If you want to play a mode with Christmas music, textures, and themes, make one and addfile it for the same effect. We're not going to force people to use such a thing, though.