The Collaborative International Dictionary of English v.0.48:

Red-letter \Red"-let`ter\ (-l?t`t?r), a.
   Of or pertaining to a red letter; marked by red letters.
   [1913 Webster]

   Red-letter day, a day that is fortunate or auspicious; --
      so called in allusion to the custom of marking holy days,
      or saints' days, in the old calendars with red letters.
      [1913 Webster]

WordNet (r) 3.0 (2006):

red-letter day
    n 1: a memorably happy or noteworthy day (from the custom of
         marking holy days in red on church calendars)