Red letter day

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.

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

[3] Red letter days for these purposes are a fixed selection of saints' days (sometimes coinciding with the traditional start or end dates of the legal terms during which sittings of the High Court take place) and of national celebrations, mostly associated with senior members of the British royal family (and, therefore, changing from generation to generation).

A comparison can be drawn with the Red Letter Days listed 100 years ago in Dress and Insignia Worn at His Majesty's Court (1921), which are on the same principle (a fixed selection of saints’ days, plus days honouring senior members of the Royal Family), except that the modern list adds the national saints of Wales and Scotland (St David and St Andrew, not listed in 1921) although not the national saint of England (St George).