Timing is everything
This refers back to the whole participation thing. If your event is spontaneous in nature and just requires people to show up at the same time and do something goofy(say, gather at a subway stop and follow the first bearded person you see as if they were Jesus), they won't need much time to prepare. The ideal time for this sort of event is at the end of the workday (between 5 and 6PM) during the week as a) the streets are more crowded and b)participants are more available. For whatever reason, Thursdays seem to be most effective.