The term originates from Medieval church calendars. Illuminated manuscripts often marked initial capitals and highlighted words in red ink, known as rubrics. The First Council of Nicaea in 325 decreed the saint's days, feasts and other holy days, which came to be printed on church calendars in red. The term came into wider usage with the appearance in 1549 of the first Book of Common Prayer in which the calendar showed special holy days in red ink.
On red letter days, judges of the English High Court (Queen's Bench Division) wear, at sittings of the Court of Law, their scarlet robes (See court dress). Also in the United Kingdom, other civil dates have been added to the original religious dates. These include anniversaries of the Monarch's birthday, official birthday, accession and coronation.
In the universities of the UK, red letter days are called scarlet days. On such days, doctors of the university may wear their scarlet 'festal' or full dress gowns instead of their undress ('black') gown. This is more significant for the ancient universities such as Oxford and Cambridge where academic dress is worn almost daily; the black undress gown being worn on normal occasions as opposed to the bright red gowns. Since most universities now only use academic dress on graduation day (where doctors wear always scarlet), the significance of scarlet days has all but disappeared.
The term "red letter day" is colloquially used to indicate any date of personal significance.
In Sweden and South Korea, a public holiday is typically referred to as "red day" (röd dag, 빨간 날), as it is printed in red in calendars.
That Was the Day That Was. BBC Wales' New Series Red Letter Day Remembers Some of the Dates That Will Go Down in Welsh History as a Turning Point for the Culture, the Employment and the Lives of People in Wales - for Better and Sometimes for Worse. These Are Six Dates That We'll Want to Remember, or That Some Would Sooner Forget
Apr 09, 2011; ESSO, MILFORD HAVEN Red Letter Day: November 3, 1960 The once thriving fishing fleet of Milford Haven was in decline in the late...