An Etymological Dictionary of the German Language/Sonnabend
Sonnabend, m., ‘Saturday,’ even in MidHG. sun-ábent, sunnen-âbent, OHG. sunnân-âband (also Samstag, OHG. sambaȥ-tac). MidHG. âbent is frequently used of the eve of a festival. In AS. 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 G. 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 orig. a week of only six days). Moreover, Sonnabend is really MidG. and LG.. —