Once, every four years we have a leap day. And no, it's not the 29^th like most would assume, but today, the 24^th:
## 2.7.1. How did the Romans number days?
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The Romans didn't number the days sequentially from 1. Instead they had three fixed points in each month:
“Kalendae” (or “Calendae”), which was the first day of the month.
“Idus”, which was the 13th day of January, February, April, June, August, September, November, and December, or the 15th day of March, May, July, or October.
“Nonae”, which was the 9th day before Idus (counting Idus itself as the 1^st day).
The days between Kalendae and Nonae were called “the 5^th day before Nonae”, “the 4^th day before Nonae”, “the 3^rd day before Nonae”, and “the day before Nonae”. (There was no “2^nd day before Nonae”. This was because of the inclusive way of counting used by the Romans: To them, Nonae itself was the first day, and thus “the 2^nd day before” and “the day before” would mean the same thing.)
Similarly, the days between Nonae and Idus were called “the X^th day before Idus”, and the days after Idus were called “the X^th day before Kalendae (of the next month)”.
Julius Caesar decreed that in leap years the “6^th day before Kalendae of March” should be doubled. So in contrast to our present system, in which we introduce an extra date (29 February), the Romans had the same date twice in leap years. The doubling of the 6^th day before Kalendae of March is the origin of the word “bissextile”. If we create a list of equivalences between the Roman days and our current days of February in a leap year, we get the following:
> 7^th day before Kalendae of March 23 February> > 6^th day before Kalendae of March 24 February> > 6^th day before Kalendae of March 25 February> > 5^th day before Kalendae of March 26 February> > 4^th day before Kalendae of March 27 February> > 3^rd day before Kalendae of March 28 February> > the day before Kalendae of March 29 February> > Kalendae of March 1 March> >
You can see that the extra 6^th day (going backwards) falls on what is today 24 February. For this reason 24 February is still today considered the “extra day” in leap years (see section 2.3). However, at certain times in history the second 6^th day (25 Feb) has been considered the leap day.
Why did Caesar choose to double the 6^th day before Kalendae of March? It appears that the leap month Intercalaris/Mercedonius of the pre-reform calendar was not placed after February, but inside it, namely between the 7^th and 6^th day before Kalendae of March. It was therefore natural to have the leap day in the same position.
So Happy Leap Day, everyone!