Question: John Sebastian is an American-born singer-songwriter, guitarist, harmonicist, and autoharpist,  best known as a founder of The Lovin' Spoonful, that did an impromptu appearance where in 1969? 
Sentence (from "Woodstock"): Scheduled for August 15–17 on a dairy farm in the Catskill Mountains of southern New York, northwest of New York City, it ran over to Monday,
Relevant evidence: No