A small group of "Critical Manners" cyclists--some in costumes--cycled from Dupont Circle to Georgetown and then through downtown Washington on the day before Halloween. Critical Manners, an alternative to the occasionally confrontational Critical Mass, ride occur on the second Friday of the month. Cyclists who participate are encouraged to obey all traffic laws such as stopping at red lights and signaling. This is the first time I've heard of a Critical Manners in DC, but similar events are held in Dayton, Boston, and Atlanta.
More DC ride coverage from the Bicycle Transportation Examiner