If you're too stressed to prep anything for your D&D group this month, WoTC and Kobold Press both had the exact same idea to give you an advent calendar of adventures for free

pcgamer.com:

Christmas is a wonderful time of year. It's full of reunions, hot chocolate, presents, and organisational stress. Most people are bogged down with travel plans, parties, presents, gift wrapping, and the like—which is a bit of a stressor for us DMs, who are already perpetually organising something for our players.

Luckily, there's going to be a wealth of free stuff to re flavour this month, since D&D and Kobold Press both happened to have the exact same idea to give away free stuff in a TTRPG advent calendar. My one-shots are saved! Also, less importantly, Christmas.

D&D's version of this is doled out via D&D Beyond's «12 days of D&D». While it is technically free, be advised you'll still have to fork over your credit card details for reasons beyond my ken. Still, at the time of writing, there are three (digital) dice sets, as well as the adventures Giants of the Star Forge, Uni and the Hunt for the Lost Horn, the first chapter of Adventures from the Radiant Citadel, and a compendium of 11 creatures from Dragonlance.

Over on Kobold Press, the charmingly-named ADVENTure Calendar has been giving adventures for both D&D 5e and the press's own Tales of the Valiant RPG—which is 5th edition compatible anyway, more like a custom homebrew rules compendium with its own subclasses than its own system. It's giving these out on Mondays, Wednesdays, and Fridays this month.

While I can't speak to the quality of this material myself—I'm a filthy homebrewer first and foremost, even if I am tempted to run a Curse of Strahd campaign—this is a lot of stuff you're getting for basically free. Even if you're not going to run these adventures or supplements rules-as-written, you can always do what I do: Chew through them like a fiend possessed and copy-paste the monster statblocks into your hastily-cobbled-together oneshot document.

D&D's freebie fest lasts until December 15, while Kobold's lasts until December 25, so you'll want to embark on the arduous task of downloading these PDFs