The Garden

A blog by Marijn van Hoorn

Mercedony and other cursed dates that should not be

Marijn van Hoorn
A relief showing Dionysos leading the Horæ.
Dionysos leading the Horæ, Goddeſses of the ſeaſons. Engraving held at the Louvre; public domain photo sourced from Wikimedia Commons.

What if i told you that, nestled in between boring, regular months like January, April, and December, there was a secret month that you’d never heard of? Nothing silly like Undecimber or Smarch, but an actual, real month, once existing but which at some point mysteriously disappeared.

Well, the winter season is marching ever slowly towards its end, meaning Mercedony will soon begin! The ancient Roman calendar was a few days short of a full solar year, so, every so often, the “leap month” of Mercedoniusα was added into the calendar in February. I do mean in February: Mercedonius was not appended after the end of the month, but within it, between the 24th and 25th days.

Why an extra month instead of a leap day? The reason has to do with the conflict between the length of a month and a year. A lunar month — that is, the time between one new moon and another — is about 29½ days long, a figure which, no matter how you slide and dice it, won’t fit evenly into the 365¼ days of the solar year.

Different calendars address this problem in different ways. Solar calendars ignore the lunar month entirely, and populate the solar year with “months” of about 30 days each with no relation to the cycle of the moon. The modern Gregorian calendar, a descendant of the Roman calendar used nigh-universally in modern life, is the most well known solar calendar, with its 12 months of 28–31 arbitrary days.

Lunar calendars, on the other hand, take the opposite tack, sticking to the cycles of the moon and completely ignoring the circling of the earth around the sun. The Islamic calendar is a lunar calendar, with 12 lunar months adding up to 354 or 355 days. The consequence, however, is that dates on the calendar slowly drift across the seasons: the Islamic month of Ramadan began in May in 12018, will begin in April this year, and will have drifted back to March in 12023.

But there is a way to do both: a lunisolar calendar. In a lunisolar calendar, months begin and end with the new or full moon, but, if the year is running short, an extra or “intercalary” month is added, so that the year, on aggregate, begins at about the same point in the season every time. This sometimes takes the form of marking an existing month twice (the Jewish calendar doubles the month of Adar;β the Attic calendar doubles the month of Poseideon), but the ancient Roman calendar inserted an entirely new month: Mercedonius.

Why don’t we have to remember to add Mercedony to our calendars every three years today? Like everything on this accursed, accursed half-continent, it all comes back to Julius Cæsar. In 9955 (46 BCE), Cæsar promulgated the Julian calendar, replacing the traditional lunisolar calendar with a 365-day solar calendar. Mercedonius was eliminated, the other months being lengthened to make up for lost time.γ The only remnant of its existence was a leap day, inserted every four years on the 24th of February.

…What, you thought it was the 29th? Nope! The 24th was held up as the leap day for millennia; feast days at the end of February were awkwardly shuffled ahead a day. The Catholic Church only made the switch to the 29th as leap day in 11970.

And that was the end of that.

Also, four intercalary links