red-letter day

noun

1. a memorably important or happy occasion

red-letter day

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.”

Word History and Origins

Origin of red-letter day¹

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

Idioms and Phrases

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]