This apparently includes the Save the Holidays event (all three Acts), and the ability to expand out into the Holiday Town expansions in your city. The Giving Tree, Santa's Sleigh and more will all vanish from the game if you've yet to complete them, so this is definitely the time to drop everything else you're doing and start working on this, if you have any desire to do so.
Unfortunately, the folks at Zynga haven't confirmed whether or not this deadline also applies to some of the timed goals in the game, like the Let's Go Sledding goal, for instance. We'll make sure to let you know if we find out just how far this deadline stretches, but for now, starting helping Santa before it's too late!
Did you finish all of the Christmas-themed content in CityVille weeks ago, or are you just now getting around to it? Do you think you'll be able to finish everything in time? Sound off in the comments.