An Etymological Dictionary of the German Language/Annotated/Sonntag

by Friedrich Kluge

Sonntag, masculine, ‘Sunday,’ from Middle High German sun-tac, sunnen-tac, Old High German sunnûntag, seems to have been even the pre-Christian term, as may be inferred from the agreement with Old Saxon sunnun-dag, Dutch zondag, English Sunday (but Old Icelandic dróttensdagr, ‘Lord's day’). Compare Montag.