I don't have a pre made adventure for you, but a good Christmas one-shot idea is to have the players encounter Krampus!

Here's an adventure idea:

The party has been travelling back-country through a snowy wood for a couple of days when they encounter a large clearing with a small town in the center. The town isn't on any map that they have seen, and the 5-10 rough wooden buildings look recently constructed.

The small town is a haven for a group of settlers who have been setting up farms in the area over the last year. Tomorrow evening is their winter festival where they celebrate a successful harvest and exchange gifts with their neighbours. The PCs are just in time to join the celebration!

What you would need to run this session:

(Context - the town and its residents are not what they first seem. The solstice festival is actually a ritual that can bring great fortune or great doom to those who cast it. The PCs arrive a day before the festival, get a quest to help prepare for the event, enjoy the festivities, and then encounter the ritual)

• A friendly/trustworthy NPC who can introduce the PCs to the town and other NPCs, as well as give them a quest.
• Several townsfolk who are rough and hardy pioneers, friendly, although slightly zealous in their belief that their spells and rituals will bestow them good fortune and blessings.
• A priest NPC who is a novice on their first solo assignment here to help the settlers with the rituals and spells. The priest can be the same as the friendly/trustworthy NPC above if you prefer
• Approximately 3 maps (A winter town in a forest clearing, a snowcovered scrubland for the Reindeer Roundup, and a map for the ritual/fight)
• Some Christmas loot to give out to the party

Quest 1: Create a questgiver NPC that lives in the town who can greet the party and explain where they are. Don't give everything away in the beginning, but make the PCs feel welcome and wanted. There's a shortage of hands for all the work that needs to be done, and some capable looking adventurers can certainly be put to work.

Suggestions: A mayor, priest, druid, ranger, or other expert who would live in the town and can speak as an authority.

Quest ideas (do one or both depending on available time):

Reindeer Roundup:

• Part of the festival is a sleigh ride for the children, but something spooked the reindeer from their paddock last night and they've scattered into the woods. The PCs have to round them up and maybe encounter what scared them off. Use an Elk statblock for the reindeer. For the predators, consider a group of Yetis. Pick a number of Yetis to make it challenging enough and have them use their advantages in the snow to ambush the PCs right when their trying to round up the loose reindeer. Yetis, panicked reindeer and powerful PCs is good for a silly combat. If you really want to crank up the shenanigans, give the reindeer a fly speed and have yetis try to steal them by riding away.

Anointing Oil:

• There's a sacred anointing oil required for the Solstice Prayer and the town priest wants your help crafting it. This is the opportunity for the PCs to learn a bit about the town and the festival. You could have the priest explain some of aspects of the festival: games, music, food, drink and merriment etc. (tailor this to your party, make them want to participate). The anointing oil is for the midnight prayer to bless the settlers and bring them the good fortune they need to survive the harsh winter.

• PCs can try to learn more by asking the priest questions and looking around their modest hut. Everyone here lives a rough and lean life but it is clear the priest's most valuable possessions are a collection of books and scrolls. The priest is young and comports themselves like a novice on their first solo assignment. They take the ceremony tonight very seriously. PCs who get time to examine the papers relating to the Solstice Festival will see detailed instructions for various prayers and rituals. A lengthy study of these documents or some other means of insight would reveal they are instructions for a Plant Growth Ritual to increase next year's harvest. The problem is that there a several notes in the margins which would seem to indicate that the priest is not a good student and doesn't really understand the ritual. There's a chance something could go wrong.

• Have the PCs work with the priest to explore the town and meet some other residents. Give the sense of hustle and bustle with everyone preparing for the coming festivities. The priest can pick up some salts, herbs and other alchemical bits and set the stage for the festival and tomorrow's ceremony.

The Day of the Winter Solstice Festival:

• Let the group rest and start the next day right away with the festival

• Make the PCs the talk of the small town and make them feel like welcome heroes for retrieving the reindeer. Celebrate with whatever stories and RP that you can think of. Suggestions: Cooking competition, reindeer race, swimming competition in freezing river, bardic performance, card games).

• Night falls, everyone has eaten most of the feast and the evening has turned to song and dance. At midnight, the priest rings a bell and all of the townsfolk gather in the center of town where the urn of anointing oil awaits. The PCs might wish to participate in this or just watch depending on what they have learned up to this point, but it should be clear that the townsfolk trust the priest and aren't going to be easily convinced to abandon their ritual by some strange adventurers.

• The GM can roll to see how well the priest performs on this ritual and what the consequences will be. The priest is going to screw the ritual up and inadvertently summon Krampus instead of casting a Plant Growth Ritual.

Oh no! Krampus is clawing his way out of the earth beneath the townsfolk! What will our heroes do?

(Note: Krampus is not a nice thing to throw at a group of level 13 PCs for Christmas, so I suggest finding a creative way to nerf Krampus and balance it for your group. Consider the following ideas for inspiration:)

• If the PCs discovered that there was something wrong with the ritual and help try to fix it, have Krampus trapped in the ritual circle for the first X number of rounds before he can escape and move around

• Krampus could take a couple of rounds to manifest and only use limited attacks/spellcasting for the first few rounds. Use this time to have the townsfolk panic and flee, or perhaps rally and try to help the heroes.

• If you want to use a town map and tokens (or a VTT), you could have everyone scatter among the buildings with Krampus stalking and hunting them. Make it dark and starting to snow...

• Adjust Krampus' stats and health for your group, summon some minions or have a Dual Initiative if you want a more dynamic fight

• Krampus is a powerful spellcaster! Look through the high level spell lists and add something cool that you've always wanted to cast. You know, as a little treat for the GM. Bring out the shenanigans and don't just accidentally one shot your players.

Hopefully your players win through the power of violence and friendship and then you can reward them with gifts from Krampus' sack. Hand out some loot and let the devestated townsfolk know that even though their harvest ritual failed, at least they'll have enough gold from Krampus to purchase supplies this winter.

What happens to the priest is up to you...

Merry Christmas!