I remember cousins who lived on Long Island talking about it in July 1969. So the event was not in any way a secret, but few people expected so many people,or such well known performers.
Janice Joplin's performance in Forrest Hills, New York during the summer of 1970 was rained out. Luckily, the next night was beautiful, and Janice sang then instead. Talented woman, gone too soon.