Update: To clarify, there is an actual festive event running in-game at the moment - Tennobaum, as mentioned after the jump. It just seems to have been somewhat sidelined by the ghoul uprising.
While every other online game worth mentioning is packing its world to the gills with seasonal cliches - snowball fights, gingerbread cookies, gaudily wrapped presents and the like - Warframe is bucking the trend and giving you a big ol' mess of angry cyborg zombie mutant clones to murder across the increasingly toxic and inhospitable plains of Cetus.
Santa Claus may want to just tick off this whole region as Too Naughty To Bother, unless the Tenno can't clean thing up over the next few weeks.
I am being at least a little bit glib. Warframe players are being given a variety of Christmas-themed goodies upon login during the season, among a few other festive bits and bobs, but the arrival of the ghouls seems rather more grim and apocalyptic than you'd expect for December. Hunting these new Ghoul enemies (Grineer clone soldiers, seemingly mutated by the toxic plains) will be the focus of the event, with NPCs around Cetus town offering contracts to stem the pseudo-undead horde, and presumably paying out some interesting new goodies.
The Ghouls themselves fall into four main categories, familiar enough to anyone who's played a zombie survival shooter - you've got the fast melee one, the sticky tongue one, the digs-under-the-ground one, and the suicidal exploding one, but given the melee-focused nature of these enemies, it might go a long way to mix up the variety of combat across the plains, which has up til' now been very ranged-oriented.
Fighting the new enemy group aside, there are some nice extra features to look forward to with this update, including a new wing opening up on your personal spacecraft; your personal quarters, customizable and free for you to decorate as you see fit with a variety of amusing space-gubbins. Presumably many of them are lurking behind in the cash shop, but considering the entirely optional nature of the chamber, that's not too big of a deal, at least in my eyes.
There are also a few new weapons to play around with, such as the Corinth shotgun. A five round magazine and solid base stats make this one look like a reliable enough weapon, although I don't see anything dethroning the double-barreled, hellaciously-powerful Tigris any time soon. For the more space ninjas looking to spice up their melee, there's the Gunsen bladed fan, attacking in graceful arcs and chargeable to do extra melee damage, a trait more commonly associated with heavier weapons.
There are also a pair of new Grineer-themed guns. The bizarrely arranged Quartakk, a quad-barreled rifle, and the Stubba machine-pistol. Unsubtle, but one thing I've always appreciated about Warframe is that each villain group is well defined by their weapons. The Grineer in particular do things loud and gracelessly, usually by throwing as much lead as possible at a problem in the shortest possible time.
You can read a bit more about the update here, which includes a short comic detailing how the incredibly shouty Vay Hek changed from being a generic NPC villain into the cyborg monstrosity he is now.