red-letter day

British  

noun

1. a memorably important or happy occasion

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red-letter day Cultural  

1. A special or memorable day; the expression refers to the old custom of printing holidays in red on calendars: “John got promoted and engaged to be married yesterday; it was truly a red-letter day for him.”

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red-letter day Idioms  

1. A special occasion, as in When Jack comes home from his tour of duty, that'll be a red-letter day. This term alludes to the practice of marking feast days and other holy days in red on church calendars, dating from the 1400s. [c. 1700]

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Etymology

Origin of red-letter day

C18: from the red letters used in ecclesiastical calendars to indicate saints' days and feasts

Example Sentences

This makes July 10, 2026, the red-letter day because taxpayers usually have three years after filing a return to make a claim disputing a penalty, according to Collins.

From MarketWatch • May 27, 2026

SpaceX’s big news, along with Anthropic’s showing potential to turn a profit far sooner than expected and OpenAI’s prepping for its public debut, made yesterday AI’s red-letter day.

From The Wall Street Journal • May 21, 2026

His 24 points - his highest tally in a Test match - included two tries, but his kicking, vision and communicative skills laid the foundation for Ireland's red-letter day.

From BBC • Sep. 10, 2023

Saturday was a red-letter day for the history of art in Southern California.

From Los Angeles Times • Jun. 21, 2022

It was a red-letter day for Elizebeth, proof that she had lived up to her old dreams of achievement.

From "The Woman All Spies Fear" by Amy Butler Greenfield