red letter

/ˈrɛdˌlɛtər/
adjective
  1. Very important or memorable, especially in a happy way.
    • Winning the championship was a red-letter moment in her career.
    • Graduation day was a red-letter occasion for the whole family.
    • The company's 50th anniversary was a red-letter event celebrated by everyone.
Antonyms
What does "red letter" mean? | whatsthatwordmean | whatsthatwordmean