In our recent D&D game, we came up with having a Hallowe'en-like celebration for our town. Thing is, our town is located pretty close to a section of land called the Chaos Scar (taken from 4e's sandbox campaign 'setting') which is responsible for much of the town's wealth.
Then I wondered... what kind of celebrations (religious, cultural, magical or otherwise) might result from citizenry who have their well-being dependent on Chaos as a literal and tangible 'force'?
Then I came to EnWorld, hat in hand. :3
So you want an organized, traditional, communal festival celebrating a force that is diametrically opposed to all of those things?
Ok, I'll try to be a little more helpful.
In general, you only see 'chaos celebrations' in communities that are traditionally very rigidly ordered. So you might have something like a 'feast of fools' where the normal social roles were relaxed, mocked, and deliberately upended. I suppose the magical equivalent would be releasing something like a Nilbog into the community, particularly if you aren't taking your setting very seriously and are ok with the gonzo.
But if you actually had an active force of chaos, it wouldn't want such celebrations to end - it would want that to be normality. If your day to day life was chaotic, you'd not have any separate and special set apart days where the community did anything collectively. There would be no way to declare a special set apart Holy Day because having special set apart days in itself implies there is some order to the universe. If you had Holy Days, no one would agree to observe them or how to observe them if they did. Holy Days would at most be relegated to "days where you do no work, if you feel like it and have sufficient wealth to make that choice". Even that wouldn't actually be the celebration of chaos, but a lingering on of an older lawful social order.
Basically, chaos days are marked by the absence of any specialness, any sense of belonging, and any meaning. Chaos wants each and every day to a be a celebration of the self and the self's desires. Setting a part a special day where we all celebrate our individuality, actually mocks the idea of individuality.