'Red-letter day' definitions:
Definition of 'red-letter day'
From: WordNet
noun
A memorably happy or noteworthy day (from the custom of marking holy days in red on church calendars)
Definition of 'Red-letter day'
From: GCIDE
- 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]