The ancient Romans were pretty good at some things, like roads, aqueducts, and long-lasting political systems, but their calendar (or calendars) were nothing we’d want to emulate. They divided the year into months, like we do, but possibly because of that unwieldy numbering system they had (out of all the systems that have been developed, it would rate somewhere around IXth), they numbered their days quite differently. They started each month normally enough; the first day was called “calends.” But when they got to the “Ides” (which was around the middle, but not always the same day), they started counting backwards from the next Calends. What we would call the 20th of a month, they’d call something like “sixth day before next calends” in a month with 30 days.
They knew about leap years, and added a day to the year every fourth year. But they didn’t add it at the end of a month, nor at the beginning. They added it six days before the end. So, in keeping with the counting-backwards idea, the days was called “bissexto” (the second sixth day). Believe it or not, that word made it into English as “bissextile,” which is still used (well, okay, not very often) in reference to a leap year or day. The word is most often found as a technical reference to such years, as in Tomlinson’s “Popular Lectures on Astronomy” from 1854: “Thus 1600 was bissextile, 1700 and 1800 were not so.”
There was another word for the extra day; it used to be called an “embolismic” day, from the Latin “embolismus” (insertion). This is also the root of the English word “embolism. “Embolismic” is pretty much obsolete, at least in reference to days or years. But the “Dictionarium Britannicum” from 1736 defines it this way: “Embolismic, intercalary.”
Besides the slightly odd idea that a dictionary for a language ought to be titled in a different language, that dictionary presents a third option for talking about leap days: “intercalary”. This one is also derived from Latin; “intercalarius” in this case. That root is a compound word made up of “inter” (between) and “calare” (to solemnly proclaim). The Roman calendar, you see, was so inconsistent and confusing that the embolismic, intercalary day wasn’t simply calculated. It had to be officially announced every time. That was how Julius Caesar got away with lengthening a year by over a hundred days, supposedly to extend the terms in office of some officials he favored. But that’s another story (that doesn’t come with any obscure words).
“Intercalary” is still in use, although again, barely. It hasn’t really changed from the way William Robertson used it in his 1783 “History of America”: “Those, which were properly intercalary days…were devoted wholly to festivity and pastime.” By the way if it seems like 1783 might have been a bit early to write a history of America, he wasn’t talking about the late English colonies; he was talking about the continent itself.
Before we criticize the Romans too harshly for their calendar, it did at least give us a bunch of new words, including “calendar”, which is from their word for the first day of a month: “calends”. Besides, our own term “leap year” doesn’t make all that much sense either. Nobody is entirely sure where the term came from, but it might be another borrowing from the Romans. The festivals they held after the intercalary day would “leap forward” by a day. And we’re still talking about the “Ides of March” (the Ides is just a day in the middle of the month, from the Latin word “iduare”, to divide). You’d always know when the Ides was approaching because although it wasn’t the same date every month, it was always nine days after the nones — which was the name for the day nine before the Ides.
The Roman calendar seems like it was more to do with vocabulary than arithmetic, which is probably a good thing. Trying to do all that stuff arithmetically would have taken more than CCCLXV days to figure out.
