The term red letter day refers to any day that has special significance. The term comes from Medieval church calendars that printed special holy days in red ink.
While the specific area of origin is not know for the phrase "red letter days" it is known that it began in medieval churches. The phrase is still used in the Uk.
A Red Letter Day is any day that is considered to be unique or special. It is a day with great significance. You could even say that your birthday is a Red Letter Day.
Red letter day is any day that is of specific significance according to the medieval church calendar. Red letter day is a synonym for a public holiday in many countries of the world.