Happy Leap Day! But why?

 Thirty days hath September,
April, June, and November.
All the rest have thirty-one,
Except for February alone:
It has twenty-eight days clear,
And twenty-nine in each leap year.

The first part of this poem is one that I recite every time I need to remember how many days are in a regular month; thirty or thirty-one. It's easy to remember February, even without the poem (which is good cause I actually needed to combine a couple traditional versions to make this one rhyme), because it's so unique.

Except it does prompt the question, "Why does February have fewer days than the rest? And why does it get an extra day every four years?"  For that, we need to look at the rest of the calendar, and a little bit of its history.

Our current calendar, the Gregorian calendar, was put into effect in October 1582 by order of Pope Gregory XIII.  Prior to this, the predominant calendar in Western world was the Julian calendar, which had been established nearly 1,600 years earlier by Julius Caesar, after whom it was named.

The Julian calendar was itself a reform of the Roman calendar system that had existed in various iterations for hundreds of years; there were originally ten months, Martius (March) through December (which means "tenth"), and a winter period where they didn't bother keeping track because it wasn't relevant to farming. At some point in the 7th century BCE, two more months were added to the end of the year: Januarius and Februarius.

At this point, there was usually an intercalary month (fancy way of saying "leap month") between February and March, and additional intercalary days were added as needed. At this point, most of the regular months had either 29 or 31 days, with February having only 28, since it was at the very end.  The official number of days in the calendar was 355, tend fewer than we have now

The Julian calendar reform, named for Julius Caesar who implemented it, lengthened the months of January, Sextilis, and December by two days, bringing them to the modern 31, and lengthened April, Quintilis, September and November by one day, bringing them to the modern 30; this brought us up to today's number of 365.  February remained at 28, but was granted an extra intercalary day every four years, bringing it to 28.  Rome also adopted January 1 as the official first day of the new year.

There were a lot of issues at first; Julius was very specific about having the first leap day (five days before the end of February, back then) in the fifth year after the calendar was introduced, and then every fourth year thereafter.  The Romans jumped the gun and started in the fourth year, and they had the leap year every three years instead.  Then they went twelve years without a leap year, and in the year 8 BCE they finally figured it out.

So now, we have a standard of 365 days in a year, with 366 days every four years, giving us an average of 365.25 days per year. Plus, the Romans eventually renamed Quintis and Sextilis to July (Iulius, after Julius Caesar)) and August (Augustus, after Augustus Caesar). That's pretty much what we have now, isn't it?

Not exactly. See, the Earth's orbit around the sun takes approximately 365.2425 days, which breaks down to 365 days, 5 hours, 49 minutes and 12 seconds (give or take a little, because nature doesn't like following human rules). So even though 365.25 is very close to 365.2425, it's still a little bit too far over. It wasn't a big deal to the ancients, cause it was good enough for them to do what they needed to do.

But then Christianity rose to prominence, and the First Nicene Council in the year 325 dictated that every Christian should celebrate Easter at the same time every year.  The date for this is determined by the "computus", a method of calculating the date which sets Easter at the first Sunday after the Paschal full moon, which is the first full moon after March 21, the fixed approximation of the spring equinox; the "full moon" in question isn't actually the full moon, but the ecclesiastical full moon, which was calculated according to the fixed 19-year Metonic cycle.

At any rate, this is where issues arose. Because the Julian calendar was slightly longer than the natural year, the date of the equinox (March 21) and the actual equinox started drifting further and further apart, which was seriously messing up the Church's ability to calculate the actual date of Easter. In 1545, the Council of Trent gave Pope Paul III to reform the calendar to bring the spring equinox back into alignment.

Over the next forty years, the computus was reformed to include a more accurate standard regarding the full moon (which I have to admit, I don't understand well enough to include here), and recalculations were made with regard to leap year.  The most significant of these is that while every four years would be a leap day, years divisible by 100 would only be leap days if they were also divisible by 400.  Hence, the years 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not, and 2100 won't be, either.

Then they removed ten days from the calendar to make up for the drift over the past several centuries, so Thursday, October 4, 1582 was followed by Friday, October 15, 1582, the first day of the Gregorian calendar. Of course, because the calendar was implemented by papal bull, it was only initially mandatory within the Catholic Church and the Papal states, which at the time were ruled by the pope.  Because it was not recognized by anyone else, it would take years for it to become widely adopted as a civil calendar; Philip II of Spain was one of the first, adopting it at the prescribed date in 1582, and because he ruled over Spain, Portugal, and much of Italy, it had a profound impact on the rest of Catholic Europe. On the other hand, the most recent country to adopt it is Saudi Arabia, who only adopted the Gregorian calendar in 2016.

In the Americas, the Gregorian calendar was adopted in New Spain and New France in 1582, while the British colonies and the future United States east of Appalachia adopted it when Great Britain adopted it in 1752 - that was only 24 years before we became a country!

Thanks to the leap day reforms, the Gregorian calendar has an error of only about one day every 3,030 years, which is pretty darn good; the 3,030th anniversary of the Gregorian calendar's adoption will be in 4612, by which time, we might have invented an even better system (actually, I think a lot of people are already working on that).

Till then, though, we get to enjoy an extra day every four years, unless it's a new century.  Hopefully those of us who are still around in the year 2100 won't be too disappointed not to have a leap day; too many is just as much of a problem as too few.

If you want even more information on this topic, check out the Wikipedia pages for the Gregorian and Julian calendars, as well as the page for the computus.