The earliest reason given for celebrating Sunday is that it is the day of the resurrection (Ep. of Barnabas, 15.9), but in the Jewish understanding of the week the first day commemorated creation and this idea was taken over even by Gentile Christians: “we assemble on the day of the sun because it is the first day, that on which God transformed the darkness and matter to create the world, and also because Jesus Christ our Savior rose from the dead on the same day” (Justin, 1 Apol. 67). From early on too, Sunday has strong eschatological overtones: it is also the eighth day “on which God inaugurated a new world” (Ep. of Barnabas, 15.8), “The image of the age to come (Basil, de Spir. Sanct. 27); it “prefigures eternal rest” (Augustine).

… from the ninth century certain saints days were allowed to take precedence over the Sunday in the West….

—Peter G. Cobb, “The calendar,” in Cheslynn Jones, Geoffrey Wainwright, and Edward Yarnold editors, The Study of Liturgy (New York: Oxford University Press, 1978), 404–05.