The Grateful Dead's shows on July 11-12, 1969, marked the opening weekend of a summer-long Music Festival at the Pavilion in Flushing Meadow Park which was created for the 1964 NY World’s Fair - all shows were $3. With little advance publicity, the Grateful Dead drew almost 5,000 fans.
During the late '60s/early '70s period, the Pavilion was actually in use, first as a cultural institution, then as a roller rink called "Roller Round." Then a few rock shows were held there in '69. The Pavilion was a unique place to listen to music, with the multi-million-dollar unisphere in plain view and a huge map of New York State painted on the floor of the Pavilion creating a surrealistic atmosphere.
The ground level of the Pavilion served as a huge round dance floor; on the balcony there were tables and chairs, the food concession (the main culinary attraction was tacos, a beautiful idea, though the reality was mediocre), and a nice view of the park. The atmosphere was totally relaxed. As in the San Francisco ballrooms, people were free to dance, crowd in front of the stage, sit in a corner, wander around, eat, or do whatever impulse dictated. There were no intrusive guards or cops.
When the Dead were about to come on, there was some squabbling between a solid block of dancers who stood in the middle of the floor -- and insisted moralistically that everyone else do the same -- and the people sitting behind them, who complained that they were cutting off the view. After a few minutes of edgy exchange, Bob Weir came onstage and announced, as the Dead do whenever they can, "The management of this place told us you can get up and dance if you want to, so why don't you get up and dance?" That did it. The dancers won; the sitters got up.
The Dead proceeded to perform for more than two hours. They played a lot of new material (notably "Don't Murder Me," a witty country-western song), some standards (including Otis Redding's "Hard to Handle," with Pigpen doing a pretty fair vocal), and several cuts from their new album, Aoxomoxoa.