The Open Dictionary

red-letter day

noun

UK/ˌredˈlet.ə ˌdeɪ/
US/ˌredˈlet̬.ɚ ˌdeɪ/

Definitions of red-letter day noun

  1. CountableUsually singular

    A significant day that is memorable, joyous, or of special importance.

    • The successful product launch was a red-letter day for our company.

    • Their wedding day was a red-letter day in their lives.

    • Receiving his first scholarship was a red-letter day for him.