Did I Mention It's Christmas?

Argyle: If this is their idea of Christmas, I gotta be here for New Year's.

—Die Hard

This is one of those fairly rare occurrences when a story takes place at Christmas time or has some heavy Chistmassy elements to it but isn't about peace, good-will to all men, the birth of Christ, Ol' Saint Nick, or any of the usual trappings of a jolly old Christmas tale. In fact, with this trope the holiday setting has no bearing whatsoever on the story in question (except maybe for some added tension that all this chaos is happening when people should be celebrating). In fact, in most of these stories if it wasn't for the occasional appearance of a Christmas tree or some other decorations or music you'd most likely forget it's Christmas at all!

Compare Soapland Christmas, which is the polar (heh) opposite of this trope. For explosive action in a Christmas-y setting, it's An Asskicking Christmas. Contrast Do They Know It's Christmas Time?.

Examples of Did I Mention It's Christmas? include:

Anime & Manga

In Mobile Suit Gundam the Battle of Solomon, featuring the iconic Mobile Armor Big Zam, takes place on Christmas Eve, though you could be forgiven for not noticing this.
- Similarly, in Gundam Wing both Zechs' attempt to blow up the Earth and the Mariemaia Army's insurrection two years later both take place around the holiday season.

Comic Books

In The Walking Dead, Dale off-handedly notes to Rick that, by counting the days, they've guessed that it is Christmas Day. Rick angrily tells him not to spread it around, as he doesn't want to disappoint his already traumatized son.

Film

Die Hard would probably be the most famous example of this Trope in that other than a few mentions here and there and the film generally being adorned with Xmas decorations its all out action and senseless violence which in a direct juxtaposition to what the seasons all about.
Eyes Wide Shut takes place during Christmas, as evident by the occasional Christmas tree in the background.
Batman Returns probably fits this trope as well.
The Sure Thing features college students during winter break... still partying it up at college and throwing a Tahitian-themed Christmas party.
Kiss Kiss Bang Bang is set around Christmas, and opens with Harry in New York while it's snowing, trying to steal a toy for his niece. The only real sign of it once the story moves to LA are some decorations and girls in Sexy Santa Dress.
The Odessa File is a political thriller about a German reporter trying to find a former Nazi concentration camp commandant in 1963. The movie opens on November 22, 1963 (the day of John F. Kennedy's assassination) with said reporter (Jon Voight) driving through streets lined with Christmas decorations as Perry Como's song "Christmas Dream" plays over the credits.
Psycho begins on "Friday, December the Eleventh" according to an onscreen graphic during the opening shot, and Christmas decorations can be seen in downtown Phoenix as Janet Leigh's character leaves town. This was a case of Throw It In, since they didn't notice there were Christmas decorations in their on-location takes until after filming had concluded, and then they just mentioned it was mid-December in the opening to properly compensate.
- First Blood is another film where Christmas decorations are visible in several scenes, although no mention of the holiday is ever made by the characters.
The Apartment takes place around Christmas and New Year's.
The Holiday takes place during the last two weeks of December, but apart from a few brief mentions, Christmas has pretty much nothing to do with the plot.
Similarly, Last Holiday does include the celebration of New Year's Eve, but isn't too heavily related to the holiday; it has more to do with the celebration of the protagonist's life, which is drawing to a very premature close (or so she thinks).
Full Metal Jacket. "Happy birthday to you, happy birthday to you, happy birthday dear Jesus, happy birthday to you."
It's a Wonderful Life is unequivocally considered one of the essential "Christmas movies," even though nearly all of it is spent showing How We Got Here and what would have happened if George hadn't been born; using the holiday as a Framing Device really only serves to make things extra warm and fuzzy for the final scene.
Brazil presents the perfect Christmas present: a decision making machine for business executives! Good to know that your odds of being fired on a whim are a solid 50/50.
The first Lethal Weapon movie takes place at Christmas, as is shown by the tree in Murtaugh's house and the complete destruction of a large stack of eggnog cartons in an action scene.
The Lion in Winter, oh so very much.

Henry II: What shall we hang... the holly, or each other?

Much of Edward Scissorhands takes place at Christmastime, but it's not a particularly Christmassy film by any stretch.
The French Connection apparently takes place at Christmas or close to it. The first time we see Popeye Doyle he's working undercover as a sidewalk Santa.

Literature

Harry Potter: Given the fact that each book takes place over the course of one school year, the narrative always at least mentions Christmas in passing.
From the Jane Austen canon, Pride and Prejudice, Sense and Sensibility, and Emma all contain scenes which take place over the Christmas season, but it has next to no bearing on any of their plots. The closest it comes is in Emma, in which it's mentioned that the bad weather gives Emma an excuse not to go to church on Christmas after the minister's uncomfortable overtures to her in the carriage a few days prior.
The novel version of The Hunt for Red October takes place during Christmastime. The only serious Christmas related event is a brief mention of Ryan going out to buy presents for his kids.
Richard Matheson's horror novel Hell House takes place between December 18 and December 24, but the only mention of the holiday comes in the very last sentence of the story, when one of the characters wishes another a merry Christmas. (The film adaptation The Legend of Hell House keeps the same datespan, but omits any reference to Christmas at all.)

Live Action TV

An early Leave It to Beaver episode has Beaver losing his barbershop money and getting Wally to cut his hair in an attempt to keep his parents from finding out. At the very end of the episode, we see the Beav wearing an angel costume and singing "Hark the Herald Angels Sing" at a school pageant...after no prior mention of Christmas in the whole episode!
The West Wing's Christmas episodes tended to be tellingly named and heavily decorated and feature a carol here and there, but the actual plots could have belonged to any episode—in contrast to the Anvilicious Thanksgiving storylines (a boatload of Chinese refugees seeking freedom to practice Christianity in the U.S.; representatives from a Native tribe staging a sit-in in the White House to protest the government's treatment of their issues).
The first part of the Doctor Who story "The End of Time" was broadcast on Christmas (and the second on New Year's). Whereas each year of the revived Doctor Who had had a very Christmassy Christmas special, this one barely mentioned it. Lampshaded with the line, "Christmas is cancelled!" partway through the first episode.
During Lost Season 4 episode The Constant the characters only find out it's Christmas Eve when they spot the date on a calendar, being too busy with much more important things.

Theatre

The entire first act of Rent takes place on Christmas Eve and into the early hours of Christmas morning. The second act starts a week later on New Year's Eve and plays out over the course of the year, with the finale once again on Christmas Eve. Aside from it needing to be winter to set up Mimi and Roger's meeting (her heat went out), the holiday doesn't play into the plot very much.

Video Games

Condemned has a level set in a mega-store which is decrepit and abandoned. From the décor and the faint ghostly music you can hear playing, it's evident that the store closed down one Christmas. This is arguably one of the most atmospheric levels of the game largely due to the out of place Xmas setting.
Parasite Eve is a great example of getting to watch the good people of Manhattan celebrate Xmas 1996 through spontaneous combustion and mass melting.
Raw Danger is a sequel to Disaster Report which is all about a city flooding and the peoples attempt to escape alive. The Xmas themes are pretty heavy with storee around the city playing xmas music and decoration being all over, plus at the start of the game you are working as a waiter at a big fancy party that has costumed santa cluses walking around entertaining the guests. Other than cosmetic themes though the fact its Christmas seems to be purely incidental.
The only indication that Snatcher takes place on December is the billboards and stock Christmas music that plays in the downtown street scene. Later on, you find one of your contacts disguised in a Santa costume.
- Hideo Kojima also did this in Policenauts, possibly as a reference to Snatcher, or perhaps to make Beyond Coast's perpetual summer more surreal to the American characters.
- According to the manual of Metal Gear 2: Solid Snake, the events of the game take place between the 24th and 25 December 1999. There's little reference to this other than the name of Holly White and her asking Snake to take her out to Christmas dinner.
Rabbids Go Home takes place in late November and early December, according to the dates on the security-cam footage. It's used as the basis for a Surprise Santa Encounter, and one Christmas Level. However, most of the work is not exceptionally Christmassy.
The PC adventure game Darkstone has absolutely nothing to do with Christmas. It therefore remains unexplained how Santa Claus ends up in the world of the game and must be rescued from one of the higher-level dungeons.
Yakuza, the first game at least is set at christmas. you have to llok pretty hard to see it but there is a few decorations and one or two store play xmas tunes.
In AMY, despite the fact that the game takes place on Christmas Eve, there's no snow and no other reference to the holiday beyond the conversation between the train conductor and Lana (who subsequently gives Amy the digital drawing pad as a present) at the beginning of the game.