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Ancient eternal cathedral calendar and clock in Messina. Sicily

Six surprising facts about the Gregorian calendar


Birthdays, holidays, or even just the weekend: without a calendar to let us know the days of the week, our lives would be in utter chaos. While most of the world is now on the same page when it comes to what day of the year, we’re on, there was a time when marking the passage of time wasn’t so straightforward.

You might not realise it, but it’s taken thousands of years of trial and error to perfect the calendar that we rely on every day of the year. But who invented it, where did it come from, and what makes a leap year? Here are six facts about why the Gregorian Calendar.

1. What is the difference between the Gregorian and Julian calendar?

Before Pope Gregory XIII instituted the Gregorian calendar in October 1582, most of the world was using the Julian Calendar. Instituted by Julius Caesar over 1,500 years previously, the Julian calendar was introduced to replace the lunar calendar.

First introduced in 45 B.C., the Julian Calendar was structured much like the Gregorian Calendar that we use today. Inclu

ding three new months in the summer, the Julian Calendar’s cycle would consist of three 365-day years spread over 12 months and then a fourth year that consisted of 366 days.

2. Why did we switch to the Gregorian Calendar?

While the Julian calendar was a fantastic solution to the previously used Lunar Calendar, it had failed to account for how many additional days it would add over time. During the Middle Ages, it became apparent that having a leap year every four years was pushing the calendar out of sync with the solar year. With no adjustments to account for this, the misalignment of the dates with the solar year was only going to get worse.

With equinoxes and solstices falling further and further away from their dates on the calendar, holidays like Easter, a celebration of spring and new life, were falling in the wrong season.

3. When was the Gregorian calendar adopted?

By 1582 the dates on calendars were out of sync with the solar calendar by as much as ten days, and with each cycle of four years only compounding the issue, Pope Gregory XIII decided it was time to address the failings of the Julian Calendar.

Eager to address the issues, Gregory authorised the creation of a new calendar that would not realign with the solar calendar, meaning that holidays like Easter would fall in spring, and the equinoxes and solstices were once again landing at the right time on the calendar.

Removing ten days from October 1582, the Gregorian Calendar was instituted and quickly adopted throughout Catholic countries.

4. It wasn’t adopted in England for 170 years

Despite the popularity of the Gregorian Calendar across most of Catholic Europe, it wasn’t instantly adopted in non-Catholic countries thanks to a deep-seated mistrust of the Catholic church and its motives. Some countries like Britain and the Americas held out for as long as 170 years before they would finally adopt the Gregorian Calendar as the standard.

Between its creation in 1582 and 1752, Europe used two calendars. Along with issues with trade, business, and law, this meant that Britain was often celebrating holidays and annual observances at different times than the rest of Europe, which made things very confusing.

5. When did we celebrate the New Year before the Gregorian calendar?

Before the introduction of the Gregorian Calendar, different countries celebrated the New Year on different dates. Some marked Christmas day as the start of the New Year, while in Great Britain and the Americas, it was celebrated on March 25th in line with the holy Feast of the Annunciation of the Virgin Mary. It wasn’t until Britain finally adopted the Gregorian Calendar in 1752 that we started celebrating the New Year at the end of December.

6. Are leap years every four years?

It’s commonly misbelieved that a leap year must fall every four years, but this isn’t strictly the case. Designed to avoid another desynchronisation between the Gregorian and the solar calendar, the Gregorian Calendar made it so that leap years were more of an adjustment year and not part of a regular cycle like in the Julian Calendar.

So how can you tell if it will be a leap year? It’s simple. If the year is evenly divisible by four, it is a leap year. For example, 2023 divided by four makes 505.75, so it’s not a leap year, while 2024 can be divided by four to make 506, which means it is a leap year.