Short answers, pulled from the story.
Apollo 4, also known as SA-501, was the first uncrewed test flight of the Saturn V launch vehicle, which launched on the 9th of November 1967. It was significant because it was the first all-up test in NASA history, meaning every stage and the spacecraft were fully functional on a single initial flight. The mission proved the Saturn V could perform, an essential step toward landing astronauts on the Moon before the end of the 1960s.
Apollo 4 launched on the 9th of November 1967 at 7:00am EST from Launch Complex 39 at Kennedy Space Center in Florida. It was the first mission ever launched from KSC and the first to use Launch Complex 39, which had been built specifically to accommodate the Saturn V.
All-up testing means every stage of a rocket and the spacecraft it carries are fully functional on the very first flight, with no incremental stage-by-stage testing. George Mueller, head of NASA's Office of Manned Space Flight, mandated this approach in a 1963 memo and it was applied to Apollo 4. The method eliminated four test missions from the Apollo schedule and was borrowed from the Air Force's Minuteman ICBM program.
Apollo 4 was delayed by a combination of contractor problems, hardware defects, and the Apollo 1 fire. North American Aviation delivered the S-II second stage six months late, and post-fire inspections uncovered 1,407 errors in the command and service module including haphazardly routed wiring. A problematic countdown demonstration test in late September and early October 1967 took three weeks instead of the expected one, and a final postponement from the 7th to the 9th of November was ordered due to concerns about Teflon seal rings degraded by Florida sun exposure.
William Donn of Columbia University described the Apollo 4 launch as one of the loudest sounds, natural or artificial, in human history, exceeded only by nuclear explosions. The sound pressure was much stronger than expected, buffeting the Vehicle Assembly Building and Launch Control Center more than five kilometers away. CBS commentator Walter Cronkite pressed his hands against a trailer window to keep it from shattering as ceiling tiles fell around him.
The Apollo 4 command module, designated CM-017, is currently on display at the Infinity Science Center in Pearlington, Mississippi, the visitor center for NASA's Stennis Space Center. It was transferred to the Smithsonian Institution in January 1969, moved to the North Carolina Museum of Life and Science in January 1974, recalled by the Smithsonian in 1984, and placed at Stennis Space Center, where it remained until 2017.