Source : Webster's Revised Unabridged Dictionary (1913)

Martinmas \Mar"tin*mas\, n. [St. Martin + mass religious
   service.] (Eccl.)
   The feast of St. Martin, the eleventh of November; -- often
   called {martlemans}.

   {Martinmas summer}, a period of calm, warm weather often
      experienced about the time of Martinmas; Indian summer.
      --Percy Smith.