Synonyms of the word 
RED-LETTER → UNFORGETTABLE
red-letter
- adj. Particularly significant or positive.
unforgettable
- adj. Very difficult or impossible to forget.
If you are interested in words, visit the following sites :
- wiktionary.org/red-letter For complete definition of a word
- 1word.ws/redletter to play with words, suffixs, prefixs, etc.