The adventdrabbles is a Holiday community that opens every year at Christmas. It's run by Ali and Torino and it's also multipairing and multifandom. Basically every day from the 1st to the 31st of December they issue a Christmas themed prompt and you can answer the challenge with a drabble. You can answer every challenge or just a few. You can build series or individual pieces. Anything goes, really. You can use any length you like for your drabbles and so on. Here are the links for the community in case you want to check them out:

IJ (which is the most active): http://asylums.insanejournal.com/adventdrabbles/

DW: http://adventdrabbles.dreamwidth.org/