Eleven months have either 30 or 31 days, while the second month, February, has only 28 days during the common year.
However, almost every four years a leap year occurs, during which an extra day—February 29—is added, extending the Gregorian calendar year to 366 days.
This extra day is also called an intercalary day.
7-Day Weeks
The days of the year in the Gregorian calendar are divided into 7-day weeks.
The weeks are numbered from 1 to 52 or 53. The international standard is to start the week on Monday. However, several countries, including the US and Canada, count Sunday as the first day of the week.
Replaced Julian Calendar
The Gregorian calendar’s predecessor, the Julian calendar, was replaced due to its significant inaccuracy. It did not correctly reflect the actual time it takes the Earth to orbit once around the Sun, known as a tropical year or solar year.
The Julian calendar’s formula for calculating leap years produced a leap day every four years, without exception. This is too often, and eventually, the Julian calendar was several days out of sync with the fixed dates for astronomical events like equinoxes and solstices.
The introduction of the Gregorian calendar allowed for the realignment with those events.
When is the next solstice and equinox?
New Leap Year Formula
At the heart of the Gregorian calendar reform was a new way of calculating leap years, which allowed for sporadic exceptions to the 4-year rule.
These are the rules we still follow today:
- Leap years are evenly divisible by 4.
- If the year can be evenly divided by 100, it is not a leap year, unless...
- ...the year is also evenly divisible by 400: Then it is a leap year.
Another alteration brought about by the Gregorian calendar was a new way of calculating the date of Easter.
The Vatican is the birthplace of the Gregorian calendar. In Italy and in a handful of other Catholic countries, it was adopted already in 1582.
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It Took 345 Years to Spread Globally
The Gregorian calendar was first adopted in Italy, Poland, Portugal, and Spain in 1582 when ten days were dropped.
It then took centuries for the new system to spread to all corners of the planet: The final country to adopt it was Turkey in 1927.
Initially, it was mainly Protestant countries in Europe that resisted the calendar reform, fearing that it was an attempt by the Catholic Church to silence their movement.
For example, England and the colonies switched over almost 200 years later when an act of Parliament introduced the new calendar, advancing the date from September 2 to September 14, 1752.
Benjamin Franklin famously wrote about the switch in his almanac:
“And what an indulgence is here, for those who love their pillow to lie down in Peace on the second of this month and not perhaps awake till the morning of the fourteenth.”
Orthodox countries followed the Julian calendar even longer, and their national churches have still not adopted the Gregorian system.
Proleptic Gregorian Calendar
If you extend the Gregorian calendar backward to dates before it was introduced in 1582, it is called the proleptic Gregorian calendar. The standard ISO 8601 requires dates before 1582 to be expressed in this format.
Who Designed the Calendar?
Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Luigi Lilio (also known as Aloysius Lilius), an Italian doctor, astronomer, and philosopher. He was born around 1510 and died in 1576, six years before his calendar was officially introduced.