When bored, you might find yourself doing weird things to kill time. Perhaps you might have opened the calendar app on your iPhone and flipped back to see what day of the week your birthday fell on the year you were born. Or maybe what day your birthday would have been in, say 1900. If you were really bored, and your birthday happened to fall on October 5th – October 14th, you might have wanted to see what day of the week it would have fallen on back in 1582, but you would have been in for quite a surprise.

Your birthday didn’t happen that year.

Of course, you weren’t born yet, but those dates didn’t happen. If, that is, you happened to be living in a country that was Catholic.

In those places (and in most places in the world today), there was no such date as October 5th, 1582. Or October 6th, 1582. Or any other date until October 15th.

But why?

Well, that is when Pope Gregory XIII decided that the Catholic Church would move from using the Julian calendar to the Gregorian calendar. This is a change that had been years in the making, thanks to tireless research done by many people at the time.

But, again… why?

Well, it would be easy to write this off as something silly that the Catholic Church did at the time, but it actually had a very good reason. While the Julian calendar was much better than most other calendars used at the time, it wasn’t perfect. The Julian calendar was created to stop the years and seasons from slowly shifting over time. To do this, the Julian calendar made it so each year was 365 days long, except every 4th year would have an extra day in it at the end of February.

This worked great for a long time, but it wasn’t perfect. Over long time periods, the seasons would slowly get out of sync. By 1582, for example, the seasons were out of sync by about 10 days. The Catholic Church saw this issue because it impacted when Easter fell. The Church calculates the date of Easter by making it the first Sunday following the first full moon after the spring equinox on March 21st. It had done things this way since the Council of Nicaea in 325 CE.

With the calendar slowly moving out of sync with the seasons, the spring equinox wouldn’t fall on March 21st, but rather March 11th. Of course, things would only get worse with each passing year, and within a few hundred more years, the seasons wouldn’t match up with the dates very well at all. So, the Church set out to solve this problem and what they came up with was the Gregorian calendar.

In the Gregorian calendar, there are 365 days in a year and a leap year every 4 years, just like the Julian calendar. If, however, the leap year is divisible by 100, then it is not a leap year; unless it is also divisible by 400, then it is.

While this may seem overly complicated, it pretty much solved the problem of the seasons and dates failing to line up properly.

Sure, in tens of thousands of years, the calendars might slowly get out of whack with the seasons again, but for now, the Gregorian calendar is an excellent system that works well. So well, in fact, that the vast majority of the world today has adopted it.

If you thought that was interesting, you might like to read about the mysterious “pyramids” discovered in Antarctica. What are they?