Chelseanacht - "night to do the impossible", basically dkmu's birthday, a time for large coordinated workings of any sort.
LS anniversary - of the first tagging, if we can find out what day that was (Ryan?) or some other significant marker, like when it was first shared on the internets. Does anyone have a link to that old forum log? I think this would have to be a day for epic tagging sprees, or else workings specifically dealing with the innards of the web.
April fools day - keep it like it is, but make the pranks magical in nature. Or do regular mundane pranks, but put *intent* behind them. Basically a day for disrupting the status quo.
Halloween - help the roaming spooks manifest more fully. Good day to open portals. How many of those characters on the street are kids in masks, and how many are real? The world shall never know. Better give them the good kind of candy.
ZaltFest - a day for nautical sorcery, adventures, intoxicants and revelry, and any combination thereof. Make an offering to Zalty on this day and you're sure to come home (or at least down) safely, no matter what risks you take. Bonus points for talking and/or dressing like a pirate. Maybe this could co-opt Talk Like a Pirate Day/September 19th?
Day to Reconsider Everything - exactly what it says on the tin. Pick a core assumption that you live with and spend the day as if it were not true. Maybe the date of this could be an individual thing, like the anniversary of a day that you Reconsidered Everything.
Space Cadet Audition Demos - a day/night to go reeeally out there, beyond your usual conditions of truth and/or usefulness, and make a serious run at something that you don't expect to grant much return.
DKMU annual masquerade ball - take on a different personality, a different style of dress and speech, different beliefs and different methods, and live it both online and off.
Artgasm - drop what you're doing and paint the world. Make something cool and post pictures/text, or go outside and make your city more spectacular. (I know this name is already in use around the US, what else could we call it?)