The Boston Celtics and the Grateful Dead. Those are two legendary American institutions that have very little in common. The one major overlap in that Venn diagram would be Bill Walton, who is revered by both Deadheads and Celtics fans alike.
According to Fred Contrada, of Masslive.com and the Springfield Republican, thanks to Walton, there were two occurrences when these cultures cross pollinated. Since Walton had penetrated the Dead scene to the point of befriending the band and crew, when the Dead played the Worcester Centrum in 1985 some of the crew came by the Boston Garden to watch him and the Celtics practice.