The Grateful Dead played a legendary gig at the Sound Storm festival on Mrs. Irene York's farm near Poynette, Wisconsin on April 26, 1970. As ever, Corry is on the case, as is LIA over at Deadsources.
I have a couple of obscure followups.
First, three years after the fact, a program called "Studio None" on cable channel 4 in Madison apparently broadcast footage of the Dead and Baby Huey from the gig. The listing refers to "the 1969 Poynette Rock Festival", but I think this must be 4/26/70. Mmmm ... wonder where that tape might be? (! listing: Wisconsin State Journal, November 26, 1973, sec. 3, p. 3.)
Second, the Little Brothers of the Poor, a lay group from Milwaukee, asked the Columbia County Board for permission to host a "Festival of Christ" on the York Farm August 6-8, 1971, expressing hope that Joe Cocker and the Dead might perform. Seems unusual entertainment for a religious festival planned to include prayer and meditation, with prospective participation by evangelist Billy Graham and Catholic Bishop Fulton J. Sheen, but Jesus Freaks will Jesus Freak, I guess. (! ref: "Charity May Hold Poynette Rock Fest," Capital Times, April 24, 1971, p. 13.)
Third, Sound Storm '72 happened in August of that year. The promoter accused advertised headliners the Grateful Dead of backing out of agreement to play the Festival, which the Dead denied, with Gail Hellund telling local radio stations and newspapers that they had never accepted Don Bobo's offer to play the gig. (! ref: Cosgrove, Howard. 1972a. Rock Fair Set to Swing. Capital Times, August 3, 29; ! ref: Cosgrove, Howard. 1972b. Poynette 'Rock Fair' Falls Apart. Capital Times, August 10, 27.)