August 15 - 1965 - The Beatles' Shea Stadium Concert
The Beatles were the biggest band in the world in 1965, and they were the only band that could have played a sports stadium. To kick of their US Tour in August 1965, they did just that by playing New York's Shea Stadium. The show was the biggest rock concert imaginable, and broke records in terms of attendance and gate receipts. The concert itself was rather poor, however, as the band could not hear themselves play and the concertgoers were far removed from the stage. Within a year, the Beatles had decided to quit touring altogether to become a studio band. So the Shea Stadium concert became the very first stadium show, while also being a harbinger of the end of the Beatles' touring.

And it is What Happened Today, August 15, in 1965.