Describing an ephemeral phenomenon, a passing fancy, a fad. Originally this expression referred to the relatively brief span of a human life. It was recorded by numerous writers and was included in James Kelly’s Scottish Proverbs of 1721. By the nineteenth century it had become a less serious thought. T. C. Haliburton (Sam Slick) included it in Wise Saws (1843): “I am a bird of passage—here today and gone tomorrow.”

You’ve done enough for one day so leave the rest until tomorrow; also, you may have lost today but you may win the next time. The first meaning, similar to Rome was not built in a day, dates at least from the sixteenth century, when it was sometimes put as tomorrow is a new day (by Lyly and others; quoted by Jonathan Swift in 1738). The exact current wording dates from the early nineteenth century, and the second meaning of that same expression from the early twentieth century. “Tomorrow’s another day,” wrote Paul Green (The Field God, 1927), and less ambiguously, Barbara Pym in A Few Green Leaves (1980), “He would probably have . . . missed his opportunity. Still, tomorrow was another day.” In the motion picture Gone With the Wind (1939), Scarlett O’Hara’s concluding line, after losing Tara and Rhett Butler, is, “After all, tomorrow is another day.”