memorable; especially important or happy: a red-letter day in his life. ... (of a Bible) having the quoted words of Jesus appear in red type: a red-letter edition;the red-letter New Testament presented at her baptism.

Why are some letters red?

Red letter edition bibles are those in which the Dominical words—those spoken by Jesus Christ, commonly only those spoken during his corporeal life on Earth—are printed rubricated, in red ink.

Where did the term red-letter day come from?

The term came into wider use in 1549 when the first Book of Common Prayer included a calendar with holy days marked in red ink; for example, Annunciation (Lady Day), 25th March, was designated in the book as a red-letter day. The term is sometimes written without the hyphen - 'red letter day'.