On 18 May 1969, Apollo 10 was launched from the Kennedy Space Center in Florida, the last mission in preparation for the Moon landing mission that was to come three months later.
Apollo 10 was a full "dress rehearsal", the only one in the Apollo program. The ship was identical to the one used for Apollo 11, and everything progressed — on board and on land — just as if a landing was going to happen. The Lunar Module was deployed on 23 May with Thomas P. Stafford and Eugene A. Cernan on board, leaving John W. Young alone in the command module, and it descended towards the Moon, spending six hours away from the Command Module and getting as close as 15.7km from the surface before going back up and docking.
The mission landed safely on 26 May on the Pacific Ocean, some 500km east of the American Samoa islands, and after that NASA was ready for the "real deal" with Apollo 11.
Commander Thomas Stafford left NASA soon after (ostensibly due to not having been selected to fly Apollo 13) and never returned to space; Young landed on the Moon with Apollo 16 in 1972 and flew the Space Shuttle's inaugural mission in 1981, among other missions; and Cernan has the distinction of being so far the last person to have been on the surface of the Moon, as a crew member on Apollo 17.