The term “red letter day” has come to mean an important or significant day. For years, printers have printed holidays in red on calendars. But the history goes back to early prayer books.
Back in the day, the Saints’ days and Christian festivals –including Holy Days of Obligation — were printed in prayer books in red. As was the custom, Sundays and Holy Days of Obligation were days devoted to the Church rather than to work.
In time, a red letter day became synonymous with a holiday from work which, of course, was a joyous occasion for most. Eventually, a “red letter day” was thought of as a day when something special happened or was expected to happen.
Historically Speaking 