Given a reference text about a red letter day, provide an explanation of what it means.

Context: A red letter day 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.

In medieval manuscripts, initial capitals and highlighted words 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 gown.

In Norway, Sweden, Hong Kong, South Korea, Indonesia and some Latin American countries, a public holiday is sometimes referred to as "red day", as it is printed in red in calendars

Answer: A red letter day is any day of special significance or opportunity such as holidays and festivals.