Red-letter Day
-Tomorrow is a red-letter day for the company; they celebrate a big anniversary.
-The couple celebrated their red-letter day at cabin in the woods.
-It's a red letter day for the country, everyone is celebrating.
Meaning: A red-letter day is any day of special significance or opportunity.
Origin: Its roots are in classical antiquity; for instance, important days are indicated in red in a calendar dating from the Roman Republic. In medieval manuscripts, initial capitals and highlighted words (known as rubrics) were written in red ink.
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Other examples:
1. Before him, no one received a proper education in his family. Therefore, the day he went to study in the university has become a red-letter day for the family.
2. His girlfriend kept inventing and celebrating so many "red-letter days" until finally he got fed up and threatened to break up with her.