Apollo 6, launched on April 4, 1968, was the Apollo program's second and last unmanned test flight of its Saturn V launch vehicle. While the vehicle experienced a number of significant malfunctions, the flight nonetheless provided NASA with enough confidence in the Saturn V to proceed to manned launches.
This was the final qualification flight of the Saturn V before its first manned flight (with Apollo 8). (While Apollo 7 was the first manned Apollo mission, it used the smaller Saturn IB, not the Saturn V.) It was also the first mission to use High Bay 3 in the Vertical Assembly Building (VAB), Mobile Launcher 2, and Firing Room 2. Another objective was testing the Command Module re-entry system under extreme conditions, simulating a worst-case return from the Moon. This objective was not met due to J-2 engine failures.
There was little press coverage of the Apollo 6 mission mainly because on the same day as the launch, Martin Luther King, Jr. was shot and killed in Memphis, Tennessee, and President Johnson had announced he would not seek reelection only five days before.