10 Days That Disappeared

In October of 1582, Europe had a problem.

Knowing when the seasons would change was incredibly important for farmers and devout Catholics alike. For farmers, the spring equinox represented the moment when day and night were the same length. This meant that there would be more and more light from here on out, and for as long as anyone could remember, it signaled the beginning of the main growing season.

The risk of losing crops due to frost went down significantly as the soil began to warm for the year. Farmers knew which seeds to plant before the equinox, and which to plant after. Naturally, festivals around this day sprung up all over the ancient world.

Meanwhile, the Church held immense power and influence in late medieval and early modern Europe. Popes often acted as advisors to kings and emperors, and the Church itself held vast territories (like the Papal States) and could wield military might.

It was incredibly important to them to get the dates for Easter right. The date of Easter is defined as the first Sunday after the full Moon that occurs on or after the spring equinox.

A drifting equinox threw off the entire Easter calculation, and it was getting worse over time.

All of this was due to Europe’s widespread adoption of the Julian calendar, so named after Julius Caesar. Caesar had noticed that the calendar the Romans were using was awful, far worse and more noticeable than what I’ve described in early modern Europe.

The Old Roman Calendar was based on the lunar cycle, so each month was 29 or 30 days. Twelve months left a gap at the end of the year, and those extra few days carried over to the next year. The Pontifex Maximus (Caesar’s important religious role) got to decide when to add an extra month they called Mercedonius.

Adding this month had a lot of potential political benefit for the powerful Pontifex Maximus. He could shorten a political rival’s term, or postpone the end of a friend’s. He could extract political concessions from vendors who wanted to sell during a religious festival, and he could even control when debts were due (based on the calendar).

Caesar “cleaned up the corruption” inherent in the old Roman calendar, making it impossible for the Pontifex Maximus to add an extra month for political gain. He added a leap year every fourth year to make the math add up.

So, one ancient Roman calendar led to a considerably less messed up Roman calendar, which held sway in most of Europe for 1500 years. The Julian calendar, though, was a ticking time bomb. That extra leap year every fourth year was just a hair too often, and every year, 11 minutes were added to the calendar.

By the time of Pope Gregory, the Julian calendar had drifted off by ten days. That meant the wrong Sunday for Easter, and the wrong time to plant seeds. It was a costly calendar, with increasingly worse consequences every year. It simply had to be corrected.

Naturally, these extra ten days that had built up over time needed to be swept under the rug. October 4th was followed by October 15th, 1582.

The Gregorian calendar kept those leap years intact, but introduced a more nuanced leap year system. Years divisible by 4 are still leap years, unless they are divisible by 100. There’s an exception to the exception, too: if the year is divisible by 400 (like 2000 or 1600), then it is a leap year.

This means that the Gregorian calendar gains about one day compared to the solar year over roughly 3,300 years.