You probably already know about International Talk Like a Pirate Day, but did you know about Pretend to be a Time Traveller Day?
The idea is to basically pretend you're a time traveler, from the past or a possible future (be it utopian or dystopian). There is, however, one catch: You can't directly tell anyone you're a time traveler!
My personal favorite technique: Walk up to random people and ask "What year is this?" When they tell you, get all quiet and then say, "Then there's still time!" before running off.
The fun starts on Monday, December 8th!