Jack Paar left the show in March 1962, citing the fact that he could no longer handle the load of putting on the show five nights a week. The Jack Paar Show moved to prime time (as The Jack Paar Program) and aired weekly, on Friday nights, through 1965.
As for Tonight, Johnny Carson was chosen as Paar's successor. At the time, Carson was host of the weekday afternoon quiz show Who Do You Trust? on ABC. Because Carson was under contract to ABC through September (they held him to his contract until the day it expired, prompting him to make occasional wisecracks on Who Do You Trust? about the situation- "I'd like to welcome you to ABC...the network with a heart"), he could not take over as host until October 1, 1962. The months between Paar and Carson were taken by a series of guest hosts, including Groucho Marx and Mort Sahl. The show was broadcast under the title The Tonight Show during this interregnum.