How the Gregorian Calendar Works
The story behind the calendar the world runs on, from Julius Caesar to Pope Gregory XIII
The short version
The Gregorian calendar is a solar calendar with 365 days in a regular year and 366 in a leap year. It has 12 months of varying lengths (28 to 31 days) and starts each year on January 1. It is the most widely used calendar system in the world, adopted by virtually every country for civil purposes.
If you have ever wondered why February has only 28 days, why September is not the seventh month even though its name suggests it, or why we skip leap years every hundred years (except every four hundred), read on.
A brief history
45 BC: The Julian Calendar
Julius Caesar reformed the chaotic Roman calendar, which had drifted far out of alignment with the seasons. His new system introduced a 365-day year with a leap day every four years. It was a massive improvement, but it was not quite accurate enough. The Julian calendar overestimates the solar year by about 11 minutes, which adds up to roughly one extra day every 128 years.
1582: The Gregorian Reform
By the 1500s, the Julian calendar had drifted 10 days from the actual solar year. Easter was falling at the wrong time. Pope Gregory XIII introduced a correction: he dropped 10 days from October 1582 (October 4 was followed by October 15) and refined the leap year rule. Catholic countries adopted it immediately. Protestant and Orthodox countries resisted for decades or centuries.
1752: Britain and the colonies
Britain and its American colonies did not switch until 1752, by which point 11 days had to be dropped. September 2 was followed by September 14. Legend says people rioted, demanding “Give us our eleven days!” (The riots are probably myth, but the confusion was real.)
20th century: global standard
Russia switched in 1918, Greece in 1923, Turkey in 1926. Today the Gregorian calendar is the international civil standard, though many cultures continue to use traditional calendars (Hebrew, Islamic, Chinese, Hindu) alongside it for religious and cultural purposes.
How leap years work
Earth takes approximately 365.2422 days to orbit the sun. A regular calendar year of 365 days falls short by about a quarter of a day. Without correction, the calendar would drift by one full day every four years.
The Gregorian leap year rule has three parts:
- A year divisible by 4 is a leap year.
- Except: a year divisible by 100 is not a leap year.
- Except: a year divisible by 400 is a leap year after all.
So 2024 was a leap year (divisible by 4). 1900 was not (divisible by 100). 2000 was (divisible by 400). This three-tier rule keeps the calendar accurate to within one day every 3,236 years.
The 12 months and where their names come from
| Month | Days | Name origin |
|---|---|---|
| January | 31 | Janus, Roman god of beginnings and doorways |
| February | 28 (29 in leap years) | Februa, Roman purification festival |
| March | 31 | Mars, Roman god of war |
| April | 30 | Aperire (Latin: "to open"), as flowers open in spring |
| May | 31 | Maia, Roman goddess of growth |
| June | 30 | Juno, Roman goddess of marriage |
| July | 31 | Julius Caesar (renamed from Quintilis) |
| August | 31 | Augustus Caesar (renamed from Sextilis) |
| September | 30 | Septem (Latin: "seven") — was the 7th month in the old Roman calendar |
| October | 31 | Octo (Latin: "eight") — was the 8th month |
| November | 30 | Novem (Latin: "nine") — was the 9th month |
| December | 31 | Decem (Latin: "ten") — was the 10th month |
Fun facts
The year originally started in March, which is why September through December have number-based names that are off by two.
February was the last month added to the Roman calendar. It got the leftover days.
July and August are both 31 days because Julius Caesar and Augustus Caesar each wanted the longest month.
When Britain switched calendars in 1752, people born between the old and new dates had ambiguous birthdays. George Washington officially changed his.
The Gregorian calendar will not need another correction for over 3,000 years.
Ethiopia still uses its own calendar with 13 months. The year 2026 in the Gregorian calendar is 2018-2019 in the Ethiopian calendar.
The Gregorian calendar alongside other systems
The Gregorian calendar is the global default for business, government, and everyday scheduling. But billions of people also use traditional calendars for religious and cultural life:
- The Hebrew calendar is lunisolar, used for Jewish holidays and lifecycle events.
- The Islamic calendar is purely lunar, with 354 or 355 days per year.
- The Chinese calendar is lunisolar, governing Chinese New Year, the Mid-Autumn Festival, and other traditional observances.
- The Christian liturgical calendar overlays the Gregorian calendar with seasons like Advent, Lent, and Easter.
For families and communities who observe more than one calendar, having both systems visible in a single view saves real confusion.
Build a calendar that works for you
CultureSync lets you combine Gregorian dates with Hebrew dates, Jewish holidays, Christian holidays, and your personal milestones in one printable PDF.
Build Your Calendar