On June 4, 1976, a small audience witnessed a concert that charted the course of pop music for the next decade. It happened at the Lesser Free Trade Hall in Manchester, England. Roughly 40 people showed up. A shocking percentage of attendees went on to form successful bands—it's often called the day the punk era began.
The headline performer was the Sex Pistols, who were just breaking through. Among those present, at least this set went on to form bands shortly after (as soon as the next day, in the case of the Buzzcocks):
Steven Patrick Morrissey — The Smiths Howard DeVoto and Pete Shelley — the Buzzcocks, Magazine Ian Curtis, Bernard Sumner, Peter Hook — Joy Division/New Order (initially called Stiff Kittens, then Warsaw) Mark E Smith — The Fall
Author David Nolan, who wrote the book I Swear I Was There—the Gig that Changed the World told the BBC:
"It's [an important event] because it's one of those moments in popular culture whereby you can put your finger on it and say: that was it, that was the day, that was the time, that was the year that was the precise moment when everything took a left turn. And that is the music that we’re listening to now, the clubs we have in Manchester, the way we buy records, the independent music scene, basically came out of that audience."
Here's a recreation of the concert from the movie 24 Hour Party People:
The Sex Pistols played London a month later, with brand-new openers The Clash and The Damned. (Joe Strummer, Clash frontman, had seen the Sex Pistols in March and hurriedly formed his band.) The Sex Pistols returned to the Lesser Free Trade Hall a few weeks later and played with the newly-formed Buzzcocks...this time hundreds of people attended. The era of punk had begun.