Critical Mass is a mass cycling event on the last Friday of every month in over 300 major cities. Barring any random snow storms, it seems like the weather will be great for it (not that that stops hard core Massers!).
So grab something on wheels without a motor and be prepared to meet some awesome people! The atmosphere is fun and friendly. Those who like to go fast end up at the front, and those who want a relaxed ride have the tail.
It meets at Dupont Circle at 6pm, leaves at 6:30. (Also sometimes people get together for a happy hour prior if you do a little google digging)
I think it usually lasts 1.5-2 hours (just in time to head to Cherry Blast!)