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red-letter (adj.)

"having a rubricated letter; written or drawn in red ink," late 14c., from red (adj.1) + letter (n.). A red-letter day (late 14c.) originally was a saint's day, marked on church calendars in red letters. 

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Definitions of red-letter from WordNet

red-letter (adj.)
memorable for being a special occasion;
on red-letter nights a large gathering greeted the celebrity
From wordnet.princeton.edu