Everybody knows that when you make a mixtape, you gotta kick it off with a killer track, to grab attention. The organizers of last winter’s Sundance Film Festival realized this too, and opened their festival with a banger: Summer of Soul (...Or, When the Revolution Could Not Be Televised). The documentary is about the mostly forgotten (or, maybe more accurately, mostly erased) 1969 Harlem Cultural Festival, a series of concerts that that drew hundreds of thousands of people and brought in some of the biggest names in all of music, including Nina Simone, Stevie Wonder, Mahalia Jackson, Sly Stone, Max Roach, B.B. King, and The Staple Singers, just to name a very few. Tons of footage of the festival was shot, and then lay in a basement for 50 years, only now being seen for the very first time. Which seems insane, although the reasons are depressingly believable, and here we are.