In July of 1971, a 31-year-old John Lennon celebrated the conclusion of the Imagine album recording sessions with a party like no other, held at Allen Klein's home in Riverdale. The below footage, mostly taken by friend Jonas Mekas (and included in his 1996 film Happy Birthday To John) shows party guests like Ringo Starr, Phil Spector, Allen Ginsberg, Miles Davis, Andy Warhol, Yoko Ono (of course) and many others. Check out Davis and Lennon shooting hoops at the 5-minute mark, and Andy Warhol snapping Polaroids throughout.