An Etymological Dictionary of the German Language/Sonntag

by Friedrich Kluge

Sonntag, m., ‘Sunday,’ from MidHG. sun-tac, sunnen-tac, OHG. sunnûntag, seems to have been even the pre-Christian term, as may be inferred from the agreement with OSax. sunnun-dag, Du. zondag, E. Sunday (but OIc. dróttensdagr, ‘Lord's day’). Comp. Montag.