Where does the phrase “red letter day” come from and What does it mean?

The expression “red letter day” may be traced back to the fifteenth century, though in that period the allusion was to a holy day of some sort, such as one memorializing a saint or a church festival.

The name came from the custom of using red or purple colors for marking those days upon the calendar, a custom that is still followed generally in showing the dates of Sundays and holy days by red figures on our present calendars.

From this ancient custom arose the practice of designating any memorable date as a red letter day.