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"): The Woodstock Music & Art Fair—informally, the Woodstock Festival or simply Woodstock— was a music festival in the United States in 1969 which attracted an audience of over 400,000.
Relevant evidence: Yes