This site covers the original British Beatles Albums plus analyses of the lyrics of non-album tracks.
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The Beatles Day Tripper was mainly written by John, though Paul added bits to the verses. It was Lennon who came up with the killer guitar riff.
Day Tripper was another of the Beatles songs that had a strong drug influence but escaped the censors notice (see She's a women). At the time of writing John was heavily into acid and it was his message to the world about being permanently high, whereas most people were not true to themselves and could only get high at the week ends and were just day t rippers. John used the reference to them being Sunday drivers, loving their trip out, but going back to reality on Monday mornings.
Taking away from the drug references day tripper tells the story of a girl who was out to tease the lads and not offer them what they want; a reference to not going all the way; so she was just a day tripper.