red-letter day

Từ điển Oxford Advanced Learner 8th

ˌred-ˈletter day [red-letter day red-letter days] BrE NAmE noun
an important day, or a day that you will remember, because of sth 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.