The event is Bike the Drive. The entire 15-mile stretch of Lake Shore Drive will be completely closed off to car traffic during the morning of June 9th, so that up to 10,000 cyclists will be able to cycle along the drive. Details are at http://www.biketraffic.org/bikethedrive/
I've signed up to volunteer at this event. When I found that the Chicagoland Bicycle Federation managed to coordinate with the City of Chicago the closing of the entire stretch of Lake Shore Drive to motorists, I knew I had to take part.