Gregorian Calendar

The Gregorian Calendar came into force on October 15, 1582, under the leadership of Pope Gregory XIII. The Gregorian model replaced the Julian calendar which had been established by Julius Caesar in 45 BC. Just like the Gregorian model, the 365-day Julian model followed the solar cycle.

A solar calendar shows the Earth’s position around the Sun. A Gregorian year is the time it takes for the Earth to go around the Sun, or 365.26 days. The fraction of 0.26 days is recovered every 4 years during leap years (a leap year includes a February month with 29 days instead of the usual 28). A solar year is divided into 12 months of 28 to 31 days.

Inspired by Christianity, the first date of the Gregorian calendar corresponds to the birth date of Jesus Christ, January 1 of the year Zero!

Still in force today, and used throughout the world, the Gregorian calendar serves as a reference in many areas.

A Brief Overview of the Origin of the Gregorian Calendar

The Julian Calendar

Before the use of the Gregorian Calendar, the Julian system was in force in the Roman Empire (from 27 BC to 476 AD). However, the Julian calendar had one flaw: it was too long a year, which meant it accumulated errors over time.

Julius Caesar introduced the Julian calendar model in 45 BCE. During the reign of Cleopatra, he drew inspiration from the Egyptian calendar. Based on the annual fluctuations of the Nile, the use of the Egyptian calendar made it possible to regulate agricultural work during the year. The Egyptians defined the year as the time needed for a wheat harvest.

Reform by Gregory XIII in 1582

Pope Gregory XIIIIn 1582, Pope Gregory XIII requested that the Julian calendar be reformed to correct its errors.

It was by introducing leap years (years with an extra day, February 29) that the Gregorian calendar proved to be more precise than the Julian model.

Usefulness of the Gregorian Calendar

On average, a Gregorian year lasts 365.2425 days. Today, it is the most used civil calendar in the world. Used in most countries to mark the passing of time, it serves as a template for planning daily activities, business and contracts, financial transactions, and much more. Thus, the Gregorian calendar facilitates the coordination of activities on a global scale, while providing a universal standard for calculating time.

A page from the Gregorian calendar in 1584

Religious Calendar

The Gregorian calendar is used to determine the dates of religious holidays in Christianity. Typically, Christmas and Easter celebrations follow the Gregorian model. Ascension, Pentecost, and All Saints’ Day are also significant events in the calendar. The birth and death dates of people follow those of the Gregorian calendar.

Friday the 13th and the Gregorian Calendar

Every year, the Gregorian calendar is marked by at least one Friday the 13th. Depending on the year, this particular day can appear one to three times. If you project yourself to infinity, there will always be one or more Friday the 13th in a 365-day cycle (or 366 days during leap years).

Who invented the Calendar?

Clay tablets dating from the third millennium BCE reveal that the Babylonians already used a calendar. This was based on the lunar cycle to link it to major community events. During this period, the Egyptians took inspiration from the Babylonian calendar, but improved it so that the dates better aligned with the seasons.

It was the Egyptians who invented the 365-day year, divided into 12 months of 30 days. To compensate for the lack of days compared to the solar year, the five additional days were considered feast days. In 2500 BC, according to historians, Shepseskaf, pharaoh of the 4th dynasty, was already using the 365-day calendar.

During Antiquity, the Greeks and Romans who ruled the Mediterranean basin were inspired by the Egyptian calendar. We can therefore confirm that the origin of the Gregorian calendar comes from Ancient Egypt.

Antiquity covered the period from the invention of writing around 3300 BC, to the fall of the Roman Empire in 476. It covered Europe, Western Asia, and Northern Africa. The period from 476 to 1492 was known as the Middle Ages. It was, amongst other things, following the discovery of America in 1492 that the modern era began. The French Revolution, in 1789, marked the beginning of the contemporary period which still continues today.