In the Middle Ages, simple survival meant working long and hard from sunrise to sunset, so any break, such as for a religious festival, was a very special day.
Called “holy days,” these feasts were marked on the calendar in red, giving us the expression “red letter day.”
Around the fifteenth century, “holy days” became “holidays,” meaning simply a day off work, still marked on the calendar in red.
At sittings of the Court of Law, Judges of the English High Court wear their scarlet robes on red letter days.
Red letter days are called scarlet days in universities in the UK, and on these days, doctors of the university may wear their scarlet ‘festal’ or full dress gowns instead of their black undress gown.
Many current calendars have special dates and holidays rendered in red instead of black.