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
    adj 1: memorable for being a special occasion; "on red-letter
           nights a large gathering greeted the celebrity"