minium in voluminum quoque scriptura usurpatur clarioresque litteras
vel in muro vel in marmore, etiam in sepulchris, facit.Minium (cinnabar) is employed also for writing in books; and the letters made with it being more distinct, even on gold or marble, it is used for the inscriptions upon tombs.
Pliny, Natural History, 33.122
Within most medieval books of hours, there were ecclesiastical calendars that had important holy days printed in red. This was a type of textual highlighting used to call attention to important festivals; a visual language that had long indicated significant textual features, paragraph organization, and wordplay (e.g. acrostics). The Latin word for red ochre and red coloring in general was rubrīca. As such, making a text red is called “rubrication” and influenced the original use for the word “rubric.” The practice of coloring significant dates in red is perhaps best known through the English idiom of a “red letter day.”
In antiquity, Roman magistrates and priests in the early Republic began to use red on official documents, legal headings, and particularly on the official state calendar, called the fasti. Looking at the painted late Republican Fasti Antiates Maiores (84-55 BCE from Antium and now at the Palazzo Massimo alle Terme, Rome) you can see the remnants of red paint and black paint, against what used to be white painted plaster. Red is similarly used for electoral dipinti in Pompeii, on epitaphs, and elsewhere in the ancient Mediterranean as well.
Within the Mediterranean, the Romans were not alone in the habit of using red in official writing, but they certainly codified its use to a much higher degree than the Greeks. Outside the Mediterranean, other cultures also used red lettering to indicate legitimacy and state writing. Historian Mu Ch’ien addresses this in his Merits and Demerits of Political Systems in Dynastic China (trans. Zhang 2019: 86), noting that in Tang and then Ming China, red ink was the color the emperor used to sign off on official documents. Official seals often used cinnabar ink to sign off on the authenticity of documents.
I began to reflect further on how and why red was used outside of legal documents in antiquity a few days ago, when Dan Diffendale posted a photo of a mosaic that I had only heard tale of or seen drawings of, the elusive Forma of the Via Marsala (CIL VI, 29845). It was found around the year 1872 on the Via Marsala in Rome and is a 1:16 mosaic plan of a Roman bath that was later locked away in the Antiquarium comunale del Celio.
As Dr. Diffendale notes in his tweet, the 2nd century mosaic is now on display in the new “Colors of the Romans” exhibition at Centrale Montemartini. It was the use of red lettering for the numerals within the mosaic forma (building plan) of the baths that caught my eye and reminded me that red was much more than the lettering of magistrates and emperors; the aesthetic vocabulary was also applied within art and maps.
Perhaps the need to recognize and to color code dates as significant either litterae-ly or figuratively has been on my mind as this blog celebrates its 7 year anniversary this week. In 2014, when I first posted at History from Below about hosting Shifting Frontiers in Late Antiquity, I had no idea what I was doing or where I was going as a blogger.
I am so thankful for my readers, their feedback, and the space to develop my voice as a public historian these past years––from Forbes to now Hyperallergic–– as I began to break out of traditional modes of publication in order to follow my curiosity and tangents. And that makes it my own re(a)d letter day.
History From Below 
Thank you,
and best wishes to tomorrows Christian Ascendion day and Islamic Sugar Feast.
What a resaerch experience that Chinese calendars would mark certain holidays in red as well.
Good wishes and sincerely
Bernd