Happy leap day! That thing that happens every four years where the month of February gets graced with one extra day for us to enjoy. To celebrate, I thought I’d take time to tell you a few things about how leap years work.

So why do leap years exist? Most calendars have 365 days per year, but one year on Earth takes 365.25 days. That extra quarter day means that the calendar will slowly drift out of alignment as time goes on.

While 6 hours may not sound like much when compared to an entire year, it adds up pretty quick. For example, after just 100 years the calendar would be more than 3 weeks out of sync, which causes big problems for things like farming. In the past, ancient priests would correct this error by adding extra days to the year wherever they felt like.

This was obviously very arbitrary and broke down if the priest made a mistake, or was too busy, or just forgot. To solve this, Julius Caesar, (yes, THAT Julius Caesar), changed the calendar so that instead of days being added manually, an extra day would automatically be added to the end of February every four years. This new Julian Calendar came into effect throughout Ancient Rome in the year 46 B.C.E.

This setup wasn’t perfect though. The truth is that the length of the year isn’t 365.25 days, but actually closer to 365.24 days. The Romans didn’t know this at the time, so the Julian calendar drifts 1 day out of sync every 129 years. Today, it’s 13 days behind, due to reach 14 days by the end of the century.

In 1582, once more accurate measurements had been taken, Pope Gregory XIII made some small changes to the calendar, reducing the drift from 1 day every 129 years to 1 day every 3030 years. Making it any more accurate than that isn’t really possible since the exact ratio between days and years varies slightly over time. The version of the calendar that Pope Gregory created is called the Gregorian calendar and it’s the system we still use today. Under that system, here’s how the leap year really works:

if a year is divisible by 4, (2020, 2024 etc.) that year is a leap year. But if one of those leap years is divisible by 100 (1800, 1900 etc.) then that leap year is skipped, unless that year is divisible by 400 (1600, 2000, etc.) in which case that year remains a leap year. Confused yet? The benefit to that complexity is that the next time the calendar falls out of sync won’t be until the year 4612.

So, now, each time another leap year comes around, you’ll understand why it is that way, and perhaps appreciate how things that happened hundreds or even thousands of years ago still impacts your life today.