Today is a leap day. If today is your birthday then, congratulations, you are a leapling which sounds like a joyous thing to be. February 29th is mostly added to the calendar every four years to make up for the fact that it actually takes our planet a little under 365¼ days to orbit the sun. It may seem to us that it absolutely comes round every four years without question, but that is because of the period in history we are living in. It is not actually the case. Having a leap year every four years makes our calendar drift off by about three days every four hundred years. So leap years do not happen in any year that is divisible by one hundred, unless it is also divisible by four hundred. So, for us the year 2000 was a normal leap year. The last time the leap year was skipped was 1900, which was a very long time ago, and the next one will be 2100, which needn’t trouble many of us.
This might be a little more complicated than you thought, but it is nothing compared to what the Romans had to put up with before Julius Caesar swept in and reformed the calendar. In the early days of Rome, the calendar was only ten months long. It covered the period from March to December. You can still see a remnant of this in the names of our months; September, October, November and December. Septem, octō, novem and decem being Latin for seven, eight, nine and ten. Nobody was very clear what went on in the rest of the year, where we have January and February. But as they were an agricultural people, they didn’t really need to do anything then, so it didn’t matter.
As the population became more urban, they really needed something that would cover the whole year. According to legend, the months of Ianuarius and Februarius were added by Numa Pompilius, the second King of Rome, in around the seventh century BC. This was a bit better, but it left the Romans with a year that was 355 days long, which is way too short. So, rather than having to add an extra day like we do, they occasionally had to add a whole extra leap month. That month was called Mercedonius. To make it even more confusing, it was created by lopping a few days off the end of February and cramming a few more in. So, when this happened, it gave them a year that was 377 or 378 days long.
The decision about whether the leap month was needed lay with the Pontifex Maximus who was the High Priest of Rome. He was supposed to keep an eye on the seasons and decide if they were drifting out of line but unfortunately this is not what happened. The Pontifex Maximus generally had an interest in politics as well, so he could insert the extra month to keep someone he liked, who was in a government position, in office for a bit longer. If he wanted them out quickly he could withhold Mercedonius for another year. Also he might make it a last minute decision, so you never knew if the leap month was coming or not. If you lived anywhere outside of Rome, you had little hope of knowing what day it was. Add to that the fact that Rome was often at war and might forget about Mercedonius all together for a few years and you can see how difficult it all must have been for everyone.
By the time Julius Caesar reformed the calender in 46 BC things had gone very wrong indeed. As Julius Caesar was also Pontifex Maximus he was able to add the extra month, but it wasn’t enough. He needed to make a giant leap. To bring the calendar year back into alignment with the solar year he needed to add a whole extra two months between November and December which are sometimes referred to as Undecember and Duodecember. 46 BC was 445 days long. It is called ‘the last year of confusion’.
In addition to this he sprinkled an extra ten days throughout the year, bringing the year up to a much more manageable 365 days and added a single leap day every four years. Everyone must have been extremely relieved. So relieved that, after he died in 44 BC they changed the name of Quintilis (the fifth month) to Julius instead. Which is why we now call it July.