King Arthur is the subject of a large body of British legend dating as far back as the sixth century. Anecdotes which could accurately place Arthur in a specific historical context are vague and often contradictory, but most of the Arthurian legends are set around the time the Roman Empire left Britain and the Saxons invaded. The stories involve Arthur and his Knights of the Round Table venturing forth from Castle Camelot in the quest for the Holy Grail, a sacred talisman with healing powers. Arthur may or may not have been based on a historical figure.
King Arthur was away on a campaign against the Anglo-Saxons when his kingdom was visited by a time-displaced John F. Kennedy. While Kennedy was desperate to get back home, he still laid Arthur's queen, Guinevere. Arthur returned just after they'd completed their tryst, but Kennedy was sent back to his own time before Arthur caught them.