The original 'Woodstock Music And Art Fair' opened on August 15, 1969, and ran until the 18th. On the evening of the second day, a blues singer from Port Arthur, Texas USA - Janis Joplin performed with her new backup group, the Kozmic Blues Band.
The video I picked does not show footage from 'Woodstock', but is of a song they played that night. "Kozmic Blues" may not be familiar to you if you're not a Janis Joplin fan. Have a listen to this slide-show type video, and you'll hear one of America's best.
Janis Joplin - "Kozmic Blues"