Elvis Presley, at the time when music promoters billed him as the "Folk Music Fireball" and "America's Only Atomic-Powered Singer," performed at the Cotton Bowl in Dallas on October 11, 1956 during the State Fair of Texas.
For a whopping ticket price of $1.25, 26,500 fans (or maybe 40,000, depending on the source) saw Elvis, Scotty Moore (guitar), Bill Black (upright bass), D.J. Fontana (drums) and the Jordanaires (backing vocals) perform.
The show had it all: an unseasonably hot night, a squished audience in the front, backstage hard feelings, almost fisticuffs, a stymied process server and a local media determined to dissect Elvis's every step and word.
Nevertheless, the seven musicians made it to showtime. Elvis—wearing "a green tweed jacket, a blue tie, a white silk shirt with pleats, charcoal trousers, a black-and-gold cummerbund and white buckskin high-topped shoes with red heels"—launched the show by driving his 1956 Candy Purple Cadillac Eldorado "Biarritz" convertible—which sported white upholstery and purple-dyed fur on the floor—through the audience. Reaching the stage, he and the band opened with "Heartbreak Hotel."
After that show, Elvis didn't perform again in Dallas until November 13, 1971 at the Memorial Auditorium.
Here's an authoritative "fan"-tastic article about this performance by Alan Hanson from the Elvis History Blog. It's the primary source for much of what you've been reading. Be sure to check out the image of a handbill featuring Big Tex banging a bass drum for the Elvis show.
Drummer D.J. Fontana gives an amazing interview about his early years with Elvis to the Musicians Hall of Fame. He recalls the 1956 Cotton Bowl show as the largest ever played by the original quartet.
Every minute's a gem. D. J.'s Cotton Bowl show recollections start at around the 9:20 mark. In his interview, D.J. talks quite a bit about the "one band, one car" nature of touring in 1956.
But wait! Here's more!