At some point in elementary school, your science teacher probably explained to you that there are 365 days in a year because that’s how long it takes for Earth to complete one full rotation around the sun. What they might not have specified, however, is that it’s not exactly 365 days—it’s actually closer to 365.2421 days.
So, if we want our calendar year to begin right when Earth begins a new rotation around the sun, we have to account for (roughly) an extra quarter of a day each year, or one day every four years. History.com reports that the Egyptians had already been doing this for a while before Europe finally caught on in 46 BC, when Roman dictator Julius Caesar and astronomer Sosigenes put their heads together to come up with what we now call the Julian calendar, which includes 12 months, 365 days, and an additional “leap day” every four years on February 29.
But rounding 0.2421 up to 0.25 each year created an issue, because it didn’t quite add up to a full day every four years—and that tiny discrepancy meant that after 128 years, the calendar year ended up starting a day before Earth had completed its rotation around the sun. By the 14th century, the calendar year was starting a whopping 10 days before Earth finished its orbit.
In 1582, Pope Gregory XIII sought to correct the error by suggesting that we simply skip a leap day every so often. His Gregorian calendar, which we still use today, mandates that we omit the leap day during years evenly divisible by 100 but not by 400. For instance, the year 2000 included a leap day because it’s divisible by 100 and 400; the year 2100, on the other hand, will not include a leap day, since it’s evenly divisible by 100, but not by 400.