An Etymological Dictionary of the German Language/Annotated/Montag
Montag, masculine, ‘Monday’; with the môn of Mond without the later d; Middle High German mântac (Suabian and Bavarian mœntac, with mutation), Old High German mânatag (*mânintag?), masculine, ‘Monday’; compare Dutch maandag, Anglo-Saxon mônandœg, English Monday (*môn equivalent to ‘moon’), Old Icelandic mánadagr. The common Teutonic term for Latin dies Lunae (French lundi, Italian lunedi).