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? 
John Sebastian: John Benson Sebastian (born March 17, 1944) is an American-born singer-songwriter, guitarist, harmonicist, and autoharpist, who is best known as a founder of The Lovin' Spoonful, a band inducted into the Rock and Roll Hall of Fame in 2000; for his impromptu appearance at the Woodstock festival in 1969; and for his No. 1 hit in 1976, "Welcome Back". 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.
Woodstock