An Etymological Dictionary of the German Language/Annotated/Donnerstag
Donnerstag, ‘Thursday,’ from Middle High German donerstac, dunrestac, Old High German donarestag; compare Dutch donderdag, Anglo-Saxon þunresdœg, English Thursday, Old Icelandic þórsdagr; the day sacred to the Old Teutonic god þunar (Old High German Donar, Old Low German Thunar, Old Icelandic þórr for þōnruz); see Dienstag and Woche. A remarkable form occurs in Middle High German (Bavarian), pfinz-tac, ‘Thursday,’ from the equivalent Greek πέμπτη.