Word Origin & History
O.E. Sunnandæg, lit. "day of the sun," from sunnan, oblique case of sunne "sun" + dæg "day," from a W.Gmc. loan-translation of L. dies solis "day of the sun," which is itself a loan-translation of Gk. hemera heliou. Cf. O.N. sunnundagr, Ger. Sonntag "Sunday." Like other weekday names, not
regularly capitalized until 17c. Sunday school dates from 1783 (originally for secular instruction); Sunday clothes is from 1642. Sunday driver is from 1925.