With thanks to Joe McMichael, again, some new info can be shared.
1967 – According to the Wake Forest University, Winston-Salem NC “Howler” yearbook, Class of 1968 Simon and Garfunkel appeared on campus for Homecoming Weekend. (Likely in October 1967.)
1968 – Sat 10 February – Pittsburgh, PA, Syria Mosque
Further more I have a date in 1968 set for April 8 in 1968, while a more or less local newspaper stated that it would take places on the 13th. It is only a few days later, but still:
Maybe a reader who was there at the University of Oklahoma can tell us more?
And a review of the 1967 Oskosh show:
Paul Simon - 'Time it was...it was' 