red-letter day

Example: “The day her citizenship came through was a red-letter day.”

Origin: Entirely literal in origin. Ecclesiastical calendars, and later printed almanacs and prayer books, marked saints' days and holy days in red ink while ordinary days were set in black — a convention visible in medieval manuscripts and carried into printed editions of the Book of Common Prayer, where the practice was familiar to anyone who owned a book at all. The color did practical work: it let a reader locate the significant days at a glance on a dense page. Figurative use for any memorable occasion is attested from the 1700s, once the visual convention was widespread enough that the phrase needed no explanation. The related habit of printing the words of Christ in red in some Bibles is a separate, much later publishing convention built on the same instinct.