While it’s commonly stated that the practice of marking important dates in red didn’t begin until the Middle Ages, in fact, in ancient Rome red ink was sometimes used on calendars to identify significant dates, as well as occasionally used on important text in documents, with the underlying reason in both cases seemingly being the same as today- to make the text stand out, in the case of the calendar marking a noteworthy day.

Through the years this practice continued, and in particular with medieval scribes who used red ink in much the same fashion for initial capital letters and certain important words (called rubrics) in their illuminated manuscripts.

As writing became more widespread, others quickly adopted the use of red ink in their writing, too, as noted by John Trevisa in Polychron (1387): “we writep capital letters wip reed colour.”

Likewise, particularly important days, such as a saints’ feast or one of the holy days, were identified on medieval church calendars with the color red. It was first identified in English by William Caxton in The boke yf Enyeydos (1490): “We wryte yet in oure kalenders the hyghe festes wyth red lettres of colour of purpre.”

In 1549, the first Book of Common Prayer included a calendar with the holy days marked in red, thus spurring use of the phrase; however, the first print reference to red-letter day is not seen until 1663, when Edmund Gayton wrote in The religion of a physician: “the Red-letter daies being the Ornament of her Year.”

Using the term in reference to a secular day is first seen in The Journals of Madam Knight in which Sarah Kemble Knight (1666-1727) wrote: “Their Chief Red Letter day is St. Election, wch is annually Observed according to Charter, to choose their Govnr.”

Notably, even today in England there remains an official practice of recognizing certain special days as “Red Letter days,” such as the Queen’s birthday and some saints’ days, at least with its High Court; on these days, all High Court judges wear scarlet robes, while on ordinary days, only judges hearing criminal cases wear the long red garments.

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Bonus Facts:

• Because the rest of the calendar was written in black ink, ordinary days used to commonly be referred to as black-letter days; for example, Tobias Smollett wrote in The tars of old England. A Comedy (1757): “O! the month of November, She’ll have cause to remember As a black letter day all the days of her life.”
• The legal phrase black-letter law, meaning well-established, basic principles of law that are no longer up for debate, traces its history to at least 1831 when the US Supreme Court wrote in Jackson v. Huntington, 30 U.S. 402: “It is seldom that a case in our time savors so much of the black letter, but the course of decision in New York renders it unavoidable.”
• Interestingly, the phrase does not derive from Black’s famous dictionary or the color of the ink. Rather, black-letter refers to an old-fashioned form of type that used to be called blackletter, but today is known generally as Gothic, – a formal, difficult-to-read script that, as it was used for all legal printing well into the 18th Century, symbolized both the law and the authority of the State. In fact, even though Roman type had been adopted for most English printing during the Renaissance, legal printers purposely retained their nearly unintelligible Gothic script, at least in part because lawyers liked keeping the fundamentals of the law to themselves.[1]
• Many attribute the changeover to William Blackstone who broke with tradition and published his famous Commentaries on the Laws of England (1765-1769) in Roman type. As the Commentaries soon became the go-to treatise for lawyers of the age, it influenced others to begin publishing law books in the far easier-to-read Roman type as well; this was, perhaps, not so much because Blackstone was the premiere authority on English law, but rather because his book was far easier to decipher than other, earlier texts.