Timekeeping is difficult to do accurately, and every calendar is off by at least a few moments. Those little moments cause calendar months to become unaligned with the seasons over time. Could you imagine December in summer? Luckily, leap years keep this misalignment to a minimum.

The Gregorian calendar, which most places adhere to today, has an extra day about every four years to make up for the difference between the calendar year and the tropical year (the actual time it takes for the Earth to orbit the Sun, about 365.24 days). This concept of adding extra time to the year to close the gap between a calendar year and an actual year is not new. Some ancient calendars divided the year into twelve 30-day months, leaving them with about a week until the Earth completed its orbit. Others, like the Hebrew, Chinese, and Buddhist calendars used a lunar calendar, which left them eleven days out of sync. In these cases, it was common for weeks or even months to be added to the calendar to close the gap. Often, this extra time was used for festivals or celebrations.

In the Roman Empire, the extra time was filled by a leap month called Mercedonius, which happened within the month of February. Unfortunately, the decision of when to hold Mercedonius was decided by high-ranking elected officials who often lengthened or shortened the month for their own political gain. By Julius Caesar’s time, the Roman calendar year and the tropical year were thoroughly disharmonious. To fix this, Caesar took inspiration from the Egyptian solar calendar to create his own system. The first year under the Julian calendar was 445 days to make up for the discrepancy between the calendar year and the seasons, but after that every year was precisely 365 days with one leap day added every four years.

Unfortunately, Julius Caesar’s calculations for the length of the Earth’s orbit were slightly off, and it turned out that adding a day every four years resulted in too much time being added. By 1582 the extra time had resulted in difficulties with calculating holidays like Easter, which relied on lunar patters, and so Pope Gregory XIII stepped up to solve the problem. Pope Gregory introduced the Gregorian calendar, which was very similar to the Julian calendar excepting how it calculated when to add leap time. Under the Gregorian system, if a year was divisible by 4, it would be a leap year. However, if the year was also divisible by 100, it would be a regular year. Unless, of course, the year was also divisible by 400, in which case it would be a leap year. This is why the year 2000 was a leap year, despite it being divisible by 100.

This system isn’t entirely perfect, though. While the difference between the calendar year and the tropical year is not as pronounced in the Gregorian calendar as in other systems, it is still off by just a fraction, which will inevitably cause problems in the future. So, eventually, humans of the future will have to make another major decision regarding their timekeeping.

So, what can you do on 2024’s leap day, where we don’t have to deal with imminent calendar issues?

Many traditions have emerged from leap day that vary vastly depending on the place. In 5th century Ireland, leap day was associated with flipped gender roles, and was the only day where women could propose to men. Germany is similar in that on leap years women would put up a decorated maypole in front of their beloved’s home and dance the maypole dance (in contrast to regular years where men would put up the pole and everyone would dance). Other places are more wary of celebrating leap years. Italy views them as bad luck, and Greece believes that marriages held during leap years are doomed to end poorly.

Anthony, an American town straddling the border between Texas and New Mexico, celebrates more like ancient societies who turned leap-time into party-time. Every leap year the town holds a festival lasting for a few days to celebrate “leaplings” (people born on February 29th).

So, for this year’s leap day you could do as the Irish and German do, and step out of your comfort zone to try something new. Or, you could follow the tradition of Anthony and take some time to celebrate with your loved ones. Make sure you take this chance while you can, because the next leap year won’t be until 2028.