New Years Eve, 1972, at the Kinetic Playground
Just before Eno left the band. (I think he quit right after the tour in early to mid-1973. His famous line about it was "I was thinking about doing laundry on stage."
The second time was either late '75 or early '76. There were one of the bands on a taping of the Midnight Special, at the pavilion of the University of Chicago.