Felix Dies Bissextum! Happy Leap Day!

“What?” you’re thinking to yourself, “Leap Day isn’t until the 29th.”

Not according to the Kalendar, which follows the guidelines set forth by Julius Caesar. The ancient Romans had this odd procedure of numbering days by counting backwards from certain landmark days in the month, viz., the Kalends, the Ides, and the Nones.

The Kalends (originally a Greek term, which is why it’s spelled with a ‘k’) were the first of the month. The last day of the previous month was called pridie Kalendas, “the day before the Kalends”. The day before that was ante diem tertium Kalendas (abbreviated “III a.d. Kal.”), which actually means the third day before the Kalends. The Romans also had this odd habit of including both days at either end when counting days, probably because they didn’t have a zero. (This is why Easter Sunday is the third day after Good Friday: “on the third day, he rose again”.)

There were two important holidays in February, which Caesar didn’t want to displace. Terminalia fell on February 23rd (VII a.d. Kal. Mar.) and Regifugium on February 24th (VI a.d. Kal. Mar.), the seventh and sixth days, respectively, before the Kalends of March. If the extra day in February were the 29th, then these holidays would have been shifted to the eighth and seventh days before the Kalends. To avoid this renumbering, Caesar decreed that the extra day would be a repeat of February 24th, the sixth day before the Kalends. And so this repeated day became known as dies bissextum (basically, “the second sixth day” before the Kalends).

So how would Frederic, the Pirate Apprentice, have fared in ancient Rome, if he had been born on Leap Day? According to William Ramsay, in his notes on the selected works of Ovid, Roman lawyers decided that,

since these two days were one in the eye of the law, any person born on the inserted day was in ordinary years to consider the ‘VI Kal Mart’ as his birthday, while any person born on the ‘VI Kal Mart’ in an ordinary year was in the ‘Annus Bissextus’ to consider the former of the two days called ‘VI Kal Mart’ as his birthday.

The Church, however, saw things a little different, and ancient practice was to observe the Feast of St Matthias on February 25th in leap years.

Ramsay also decries the adjective bissextilis as a barbarism (introduced, it would seem, by the Venerable Bede), but that’s how we get the phrase “bissextile year” (which isn’t particularly common anymore).

And for completeness, we’ll say a little about the other two landmark days, the Ides and the Nones. Everybody knows that the Ides of March fall on March 15 (the day Caesar himself fell, if you’ll pardon the pun). But the Ides only fell on the 15th in four months: March, May, July, and October. In the remaining months, the Ides fell on the 13th. The Nones were the ninth day before the Ides, which means they fell on either the 7th or the 5th (counting backwards, and including both terminal days, remember). There’s a mnemonic poem to help you remember this:

In March, July, October, May
The Ides fall on the fifteenth day,
The Nones the seventh; all besides
Have two days less for Nones and Ides.

And so, Felix dies bissextum!

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