Terraria has it's Christmas themed stuff right now.
Some bunnies are in Santa costumes (sadly it does not last once the holiday rolls over), and Slimes have bows on them.
Enemies have a slight chance of dropping "presents" as well, which can be unwrapped into festive clothes, candy cane blocks, Pine blocks (which can even be made into chairs, tables and doors), decorations, and even weapons.
The wandering salesman can sell Christmas themed paintings, and the Paint will sell festive wallpaper as well.
If you're in hard mode, you can summon the Frost Legion mob to fight. You can summon it by using Snow Globes found in Presents (opened in Hard Mode worlds)
Irrelevant of the time of year, you can forge a "Naughty Present" in hardmode from Souls of Fright, Ectoplasm, and Silk to summon the Frost Moon at night. It is an endless mob that can be tougher than the final boss- so be prepared. You'll get great loot from it though.
Finally, if you have enough villagers, beat the Frost Legion before Jan 1st, and an open room, Santa can visit as a vendor, selling decorations that can be applied to trees.