Short answers, pulled from the story.
Apollo 6 launched on the 4th of April 1968 at 7:00:01 am EST from Launch Complex 39A at Kennedy Space Center. It was the second test flight of the Saturn V launch vehicle, intended to qualify the rocket for crewed spaceflight by sending a command and service module on a simulated trans-lunar trajectory and return.
Two J-2 engines on the S-II second stage shut down due to failures in the augmented spark igniter propellant lines. In vacuum, without the frost that damped vibrations during ground testing, the liquid hydrogen line to engine two's igniter failed. Excess oxygen caused a burn-through and pressure loss; a wiring error then caused engine three to shut down as well. A similar failure likely prevented the S-IVB third stage engine from restarting.
Pogo oscillation is a longitudinal vibration caused by the interaction between engine thrust fluctuations, fuel line resonance, and the rocket's structural response. During Apollo 6's first-stage burn, pogo produced a g-force of plus or minus 0.6 g on the vehicle, well above the 0.25 g design maximum. The fix involved filling valve cavities with helium gas before launch to damp the pressure oscillations.
Apollo Program Director Samuel C. Phillips called reaching orbit despite two lost engines "a major unplanned accomplishment," while George Mueller later stated it "will have to be defined as a failure." Despite the engine failures, NASA gained enough confidence in the Saturn V to cancel a planned third uncrewed test flight and proceed directly to a crewed mission on Apollo 8.
Apollo 6 launched on the same day Martin Luther King Jr. was assassinated in Memphis, Tennessee. President Lyndon B. Johnson had also announced he would not seek reelection only four days earlier, leaving the press focused on those events rather than the rocket test.
The Apollo 6 command module, CM-020, is on display at the Fernbank Science Center in Atlanta, Georgia. After the mission it was transferred to the Smithsonian Institution before being placed on permanent exhibit there.