An Etymological Dictionary of the German Language/Fastnacht
Fastnacht, f., ‘Shrove Tuesday,’ from MidHG. vasenaht, ‘eve of the first day of Lent.’ According to the OTeut. computation of time (comp. Abend) the evening and night were counted as part of the following day (thus in AS. frîgeœ̂fen, ‘Thursday evening,’ frîgeniht, ‘Thursday night’). The meaning given above did not belong to the word originally. The first part of the compound is an old verb faseln, ‘to play the fool’; the form Fastnacht may have been introduced by the priests.