In The Beatles Anthology book, Paul McCartney said he had asked a chauffeur – who had driven him to John Lennon's house in Weybridge, "How have you been?" The chauffeur replied, "Working hard. Working eight days a week." (Paul's license was suspended because he had gotten a speeding ticket.) Paul mentioned the phrase to John and they immediately started writing the song. The Beatles should have paid royalties to that chauffeur!
Many sources attribute the phrase "a hard day's night" to Ringo Starr. In a 1964 interview with Dave Hull, who hosted 9 PM to midnight at Pasadena top-40 station KRLA and was known as "The Fifth Beatle" (along with countless others!), Ringo explained, in reference to filming the A Hard Day's Night movie, "We’d worked all day and we happened to work all night. I said, ‘It’s been a hard day’ – and I looked around and saw it was dark so I said, 'hard day's night.'" But John Lennon's In His Own Write, published on March 23, 1964, includes the line "He'd had a hard day's night that day" in the poem "Sad Michael." The movie was filmed in March-April 1964, so that Ringo origin story is obviously erroneous. Lennon did say he got the phrase from Ringo – but when, and in what context, nobody seems to know.