Recently Thonas Tomasi mentioned that he saw Simon & Garfunkel perform during the Winter Carnival of 1967 in Johnson (VT) at the Johnson State College. Probably in February 1967, but so far no definitive confirmation has been found.
But, in an article of their upcoming show in Burlington (1967 March 12) it is mentioned.
Read here the (1967-03-08) St. Albans Daily Messenger article:
Someonean exact date for the Johnson State College show?
Update:
Simon & Garfunkel appeared on December, 11 1966 at Johnson State College:
Paul Simon - 'Time it was...it was' 
They did play Johnson State College. My father was there during that time and told me about it.
I attended JSC starting Fall of ’67 and was told several times about this
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Hi, thanks for your comment. It seems it was on December 11, 1966: https://www.setlist.fm/setlist/simon-and-garfunkel/1966/johnson-state-college-johnson-vt-23a484d7.html There was also mentioning of it in the JSC Yearbook “Janus”. I will add info to the post.