Answer by taking a quote from the following article:

Charles "Pete" Conrad Jr. (June 2, 1930 - July 8, 1999), (Captain, USN), was an American NASA astronaut, aeronautical engineer, naval officer and aviator, test pilot, and during the Apollo 12 mission became the third man to walk on the Moon. Conrad was selected in NASA's second astronaut class. He set an eight-day space endurance record along with his Command Pilot Gordon Cooper on his first spaceflight, the Gemini 5 mission. Conrad also commanded the Gemini 11 mission.

Conrad was assigned in December 1966 to command the backup crew for the first Earth orbital test flight of the complete Apollo spacecraft, including the Lunar Module (LM) into low Earth orbit. Delays in the LM's development pushed this mission to December 1968 as Apollo 8. But when one more delay occurred in readying the first LM for crewed flight, NASA approved and scheduled a lunar orbit mission without the LM as Apollo 8, pushing Conrad's backup mission to Apollo 9 in March 1969. Director of Flight Crew Operations Deke Slayton's practice was to assign a backup crew as the prime crew on the third following mission. If the swap of 8 and 9 had not occurred, Conrad might have commanded Apollo 11, the first mission to land on the Moon. On November 14, 1969, Apollo 12 was launched with Conrad as Commander, Dick Gordon as Command Module Pilot, and Alan Bean as Lunar Module Pilot. The launch was the most harrowing of the Apollo program, as a series of lightning strikes just after liftoff temporarily knocked out power and guidance in the Command Module. Five days later, after stepping onto the lunar surface, Conrad joked about his own small stature by remarking: Whoopee! Man, that may have been a small one for Neil, but that's a long one for me. He later revealed that he said this in order to win a bet he had made with the Italian journalist Oriana Fallaci for $500 to prove that NASA did not script astronaut comments (Fallaci was convinced that Armstrong's "One small step for man" speech had been written for him and were not his own words). (In actuality, Conrad's "long one" and Armstrong's "small step" refer to two different actions: going from the ladder down to the landing pad, then stepping horizontally off the pad onto the lunar surface. Conrad's words for stepping onto the Moon were "Oooh, is that soft and queasy.") One of the photos that he took during the mission with his own image visible on the helmet visor of Al Bean was later listed on Popular Science's photo gallery of the best astronaut selfies.

What was Conrad's involvement in the Apollo program?
Conrad was assigned in December 1966 to command the backup crew for the first Earth orbital test flight of the complete Apollo spacecraft,