American English

Definition of red-letter day noun from the Oxford Advanced American Dictionary

 

red-letter day

 noun
noun
 
 
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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 Winning the championship was a red-letter day for our team. From the custom of using red ink to mark holidays and festivals on a calendar.
See the Oxford Advanced Learner's Dictionary entry: red-letter day