
September 17, 2011 marked the official start of Occupy Wall Street. Thousands of people participated, and a few hundred camped overnight.
Sources:
Photos taken on September 17 (Flickr)
Occupy Wall Street blog
US Day of Rage (one of the organizing groups)
General assembly minutes
The Dissenter live blog
Democracy Now coverage
Bloomberg Businessweek article on David Graeber (one of the OWS activists)
Salon article
NYTimes blog
Take the Square blog

