English

Definition of red-letter day noun from the Oxford Advanced Learner's Dictionary

 

red-letter day

 noun
noun
BrE
 
; NAmE
 
 
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an important day, or a day that you will remember, because of something good that happened then birthdays, wedding anniversaries and other red-letter days It was a red-letter day for United, who won a place in Saturday’s final. From the custom of using red ink to mark holidays and festivals on a calendar. Word Origin early 18th cent.: from the practice of highlighting a festival in red on a calendar.
See the Oxford Advanced American Dictionary entry: red-letter day