Memorable, special (occasion) (3-6)

I believe the answer is:

red-letter

(Other definitions for red-letter that I've seen before include "Scarlet missive to signal memorable day" , "This day is memorable due to scarlet missive" , "This day is memorable - hence scarlet missive" , "(Sign of day that's) memorably important" .)

I've seen this clue in the Sydney Morning Herald.

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