Today is a leap day. There's one every four years, as you know. Some of you will know the rest of this as well, but when did that ever stop me?

Every 1461st day is a leap year, which means an estimated 273 Bristolians celebrate their birthday today. Like many things, it all goes back to Roman times.

Before Julius Caesar reorganised the Roman year, it was a right old mess. Months were originally lunar (the famous Ides, as in the Ides of March, were days with a full moon, whereas new moon days were called calends, giving us the word calendar), but by the time of written records they'd realised that lunar months and solar years didn't fit neatly together. They'd settled on ten months, with about sixty days that weren't in the official calendar at all. Because the precise details were controlled by the priests, and varied every year, there was confusion across the Empire.

Which was getting to be a big place. Caesar added Gaul (France and Belgium, roughly speaking), but even before that there was Italy, Spain, much of the north African coast, most of the Balkans, the whole of Greece and western Turkey. The Romans were changing from provincial warlords to imperial administrators, and they needed a better calendar.

It was also a religious problem. The Roman religion had nearly as many holy days as the Catholics, and each of them had set rites, and so on. Imagine not being quite sure if it was Christmas. If you ask me it goes on quite long enough as it is, without several weeks when it might be Christmas or it might not.

At least they didn't have to worry about the football. Which was a good thing, because if they'd had a Champions League in those days you'd have had hordes of Olympiakos fans turning up in Rome weeks late for the Lazio game. Or worse, weeks early, and hanging about round the Circus Maximus, looking for trade.

In the end they changed the number of months in the year to twelve. This is why September, October, November and December, the ninth, tenth, eleventh and twelfth days in our calendar, have names originally derived from the Roman numbers seven to ten - septem, octo, novem and decem. They still had problems, though, because they only allowed for 355 days a year. The rest they just added in every few years whenever they felt they needed some. This meant that people far away from Rome often still thought it was an entirely different day from the day it was actually supposed to be. As well as religious rites, this affected taxation, and if there was one thing the Romans felt even more strongly about than properly ordered religious rites it was taxes.

So Julius Caesar decided it was time to sort it all out. The first thing he did was to make 46BC 445 days long, to make up for all the days they'd misplaced. Then he increased the length of some months to bring the whole thing up to 365 days, and added an extra day every four years.

After his death they honoured his memory by naming the seventh month Julius (July today, just as August was renamed for Augustus), but then unfortunately rather dishonoured it by bollocksing up the leap years. Because they misunderstood the concept of every four years, imagining that you counted from last leap year to this one inclusively, they ended up with one leap day every three years instead. It was put back to every fourth year in the reign of Augustus.

And for a while, everything was fine. Unfortunately, the year isn't exactly 365 1/4 days long. It's actually 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. This means that Caesar's years were all an average of 10 minutes and 48 seconds too long.

This may not seem like much, but it adds up over time. After 134 years, you've lost a day, and each 134 years after that you've lost another one. It's annoying for Catholics, who want to be sure that it really is the Sacred Rite of the Holy Motherlode after all, and not actually the Feast Day of the Blessed Twatface, Patron Saint of Ford Capris and Monopoly.

And you'd better believe it matters. When a seventh century Saxon king massacred hundreds of Celtic monks in the seventh century the Venerable Bede said that this was God's punishment for their refusal to submit to the authority of Rome. He called them faithless Britons, who had rejected the offer of eternal salvation and were incurring the punishment of temporal destruction.

Their crime? To celebrate Easter one day early.

As is almost a religious tradition, stupid reasons led to a useful innovation. In 1582, Pope Gregory acted to solve the problem. He introduced a new rule, according to which the leap year was skipped every 100 years, but not skipped every 400. That's why 1900 wasn't a leap year, but 2000 was.

Humanity gets there, in a roundabout kind of way, and Gregory's astronomers had done a remarkably good job. It's accurate to within 26 seconds a year, or 2  hours, 53 minutes and 23 seconds every four hundred years - a complete Gregorian cycle. Eventually we'll have to add on another day, but that won't be until the year 4904. Unfortunately, that's a leap year as well, so they'll have to add two extra days.

Two days in the same year? Really, they could have thought on.