Woodstock is a 1970 documentary film of the watershed counterculture Woodstock Festival which took place in August 1969 near Bethel, New York. Entertainment Weekly called this film the benchmark of concert movies and one of the most entertaining documentaries ever made. The 1970 theatrical release of the film ran 185 minutes. A director's cut spanning 225 minutes was released in 1994. An expanded 40th Anniversary Edition of Woodstock , released on June 9, 2009 in Blu-ray and DVD formats, features additional performances not before seen in the film. In 1996, Woodstock was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant". (More from Wikipedia)
Since Richie Havens was about the only musician who had arrived at the 1969 Woodstock festival before the highways became hopelessly jammed, he played for hours. As he recalls (quoting from Wikipedia): "I'd already played every song I knew and I was stalling, asking for more guitar and mic, trying to think of something else to play – and then it just came to me . . . The establishment was foolish enough to give us all this freedom and we used it in every way we could."
After a remarkably quick guitar tuning, Richie Havens then improvised a song called "Freedom" that was based on the Negro spiritual "Sometimes I Feel Like a Motherless Child". It was this performance that made it into the Woodstock movie. Bob Dylan has incorporated "Motherless Child" into his songs; Wikipedia lists dozens of others who have also recorded the song in one form or another.