The Collaborative International Dictionary of English v.0.48:

Michaelmas \Mich"ael*mas\, n. [Michael + mass religious service;
   OE. Mighelmesse.]
   The feast of the archangel Michael, a church festival,
   celebrated on the 29th of September. Hence, colloquially,
   autumn.
   [1913 Webster]

   Michaelmas daisy. (Bot.) See under Daisy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Michaelmas
    n 1: honoring the archangel Michael; a quarter day in England,
         Wales, and Ireland [syn: Michaelmas, Michaelmas Day,
         September 29]