An Etymological Dictionary of the German Language/Annotated/Sonnabend
Sonnabend, masculine, ‘Saturday,’ even in Middle High German sun-ábent, sunnen-âbent, Old High German sunnân-âband (also Samstag, Old High German sambaȥ-tac). Middle High German âbent is frequently used of the eve of a festival. In Anglo-Saxon the corresponding sunnan-œ̂fen is used only of the ‘eve of Sunday.’ It follows from what has been said under Fastnacht that the name of a part of the day was in German applied to the whole day. According to the article Samstag, a native term for Saturday seems to have been wanting among the Teutons (perhaps they had originally a week of only six days). Moreover, Sonnabend is really Middle German and Low German. —