Definify.com
Webster 1913 Edition
Red-letter
Red′-letˊter
(-l?tˊt?r)
, Adj.
Of or pertaining to a red letter; marked by red letters.
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.