* * * * *
Leap Day … it's not the day you think it is
I don't recall exactly how the conversation turned towards leap days, but it
did. I think it may have been something to do with birthdays and being born
on February 29^th, the 29^th being the leap day once every four years.
“But it's not,” I said. “It's actually February 24^th.”
“How is it the 24^th and not the 29^th?” asked Spring [1].
“It has something to do with the Roman calendar,” I said. “But I'll have to
find it on the Calendar FAQ (Frequently Asked Questions) [2].”
And here it is, from the Calendar FAQ, § 2.7.1:
> 2.7.1. How did the Romans number days?
>
> * * * * *
>
> 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 there you go … February 24^th is the leap day, not the 29^th.
[1]
http://www.springdew.com/
[2]
http://www.tondering.dk/claus/cal/calendar26.txt
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