A red letter day (sometimes hyphenated as red-letter day or called scarlet day in academia) is any day of special significance or opportunity. Its roots are in classical antiquity; for instance, important days are indicated in red in a calendar dating from the Roman Republic (509–27 BC).^[1]

In medieval manuscripts, initial capitals and highlighted words (known as rubrics) were written in red ink. The practice was continued after the invention of the printing press, including in Catholic liturgical books. Many calendars still indicate special dates, festivals and holidays in red instead of black.

In the universities of the UK, scarlet days are when doctors may wear their scarlet 'festal' or full dress gowns instead of their undress ('black') gown.

In Norway, Sweden, Hong Kong and South Korea and some Latin American countries, a public holiday is sometimes referred to as "red day" (rød dag, röd dag, 빨간 날, 紅日), as it is printed in red in calendars.^[2]