Answer by taking a quote from the following article:

Knievel was born on October 17, 1938, in Butte, Montana, the first of two children of Robert E. and Ann Marie Keough Knievel. His surname is of German origin; his paternal great-great-grandparents emigrated to the United States from Germany. His mother was of Irish ancestry. Robert and Ann divorced in 1940, after the 1939 birth of their second child, Nicolas, known as Nic.

ABC's Wide World of Sports was unwilling to pay the price Knievel wanted for the Snake River Canyon jump, so he hired boxing promoter Bob Arum's company, Top Rank Productions, to put the event on closed-circuit television and broadcast to movie theaters. Investors in the event took a substantial loss, including promoter Don E. Branker, as well as Vince McMahon of the World Wrestling Federation. Arum partnered with Invest West Sports, Shelly Saltman's company, to secure from Invest West Sports two things: first, the necessary financing for the jump, and second, the services of Saltman, long recognized as one of America's premier public relations and promotion men, to do publicity so that Knievel could concentrate on his jumps. Knievel hired subcontractor and aeronautical engineer Doug Malewicki to build him a rocket-powered cycle to jump across the Snake River, and called it the Skycycle X-1. Malwecki's creation was powered by a steam engine built by former Aerojet engineer Robert Truax. On April 15, 1972, the X-1 was launched to test the feasibility of the launching ramp. The decision was then made to have Truax build the Skycycle X-2 and have it take off and fly more like a rocket than a motorcycle.  The launch took place at the south rim of the Snake River Canyon, west of Shoshone Falls (42.597degN 114.423degW / 42.597; -114.423), on Sunday, September 8, 1974, at 3:36 p.m. MDT. The steam that powered the engine was superheated to a temperature of 500 degF (260 degC). The drogue parachute prematurely deployed as the Skycycle left the launching rail and induced significant drag. Even though the craft made it all the way across the canyon to the north rim, the prevailing northwest winds caused it to drift back into the canyon. By the time it hit the bottom of the canyon, it landed only a few feet from the water on the same side of the canyon from which it had been launched. If he had landed in the water, Knievel likely would have drowned, due to a jumpsuit/harness malfunction which kept him strapped in the vehicle. He survived the jump with only minor injuries.  Since the 1974 launch, seven daredevils have expressed interest in recreating the jump, including Knievel's two sons, Robbie and Kelly. In 2010 Robbie announced he would recreate the jump. Stuntman Eddie Braun announced he was working with Kelly and Robert Truax's son to recreate the jump using a replica of the Skycycle X-2. Braun's jump took place on September 16, 2016, and was completed successfully.

when was the launch?
on Sunday, September 8, 1974, at 3:36 p.m. MDT.