Where Our Calendar Comes From: Months, Leap Years and Moving Holidays
The calendar on your wall is a 2,000-year-old Roman design, corrected by a pope, and adopted country by country over more than three centuries. That history explains its oddities: months of different lengths, names that don’t match their numbers, and holidays that wander from year to year.
Need a calendar? Make one with the printable calendar, with holidays from the US, UK, Canada and major religions.
The Roman calendar
The earliest Roman calendar, according to tradition, had only ten months, starting in March, with the winter left out altogether. That’s why the last four months have number names that no longer fit:
| Month | Latin root | Meaning | Position today |
|---|---|---|---|
| September | septem | Seven | 9th |
| October | octo | Eight | 10th |
| November | novem | Nine | 11th |
| December | decem | Ten | 12th |
January and February were added later, traditionally by King Numa around 713 BC. January is named after Janus, the two-faced god of doorways and beginnings, and February after Februa, a Roman festival of purification.
Why February is so short
Numa’s calendar followed the Moon, giving a year of about 355 days. Romans considered even numbers unlucky, so most months had 29 or 31 days. One month had to be even for the total to work, and February, already the month of rituals for the dead, got 28 days.
To keep the calendar in line with the seasons, officials added an extra month every few years. The trouble was that they decided when, and sometimes added it to keep allies in office longer. By the time of Julius Caesar, the calendar was about three months out of step with the seasons.
Julius Caesar’s fix: the Julian calendar
In 46 BC, on the advice of the astronomer Sosigenes of Alexandria, Julius Caesar replaced the lunar calendar with one based on the Sun:
- A year of 365 days, with the extra days spread across the months to give the lengths we still use.
- One extra day every four years: the first leap year.
To catch up with the seasons, 46 BC itself was stretched to 445 days, the longest year in history, sometimes called the “year of confusion”.
After Caesar’s death, the month Quintilis (“fifth”) was renamed July in his honor, and later Sextilis (“sixth”) became August for the emperor Augustus.
There’s a popular story that Augustus took a day from February so that his month would be as long as Caesar’s. It isn’t true: August already had 31 days under Caesar’s reform. The story was invented in the Middle Ages.
Pope Gregory’s correction
The Julian year averages 365.25 days, but the real year, from one spring equinox to the next, is about 365.2422 days. That 11-minute difference adds up to a whole day every 128 years or so. By the 1500s the calendar was about ten days out, and the spring equinox, which the church used to set the date of Easter, was falling around March 11 instead of March 21.
In 1582 Pope Gregory XIII introduced a fix in two parts:
- Skip ten days, once. In Catholic countries, Thursday 4 October 1582 was followed by Friday 15 October.
- Change the leap year rule. A year is a leap year if it divides by 4, except century years, which are leap years only if they divide by 400.
So 2000 was a leap year, but 1900 wasn’t, and 2100 won’t be. That drops three leap days every four centuries, giving an average year of 365.2425 days, close enough to drift only about one day in 3,000 years.
Country by country
Catholic Spain, Portugal, Italy and Poland switched straight away, but Protestant and Orthodox countries were in no hurry to follow the pope. For a long time, the date depended on which country you were in.
- Britain and its colonies, including the future United States, switched in 1752, by which time they had to skip 11 days: Wednesday 2 September was followed by Thursday 14 September. The same law moved New Year’s Day from 25 March to 1 January.
- Japan adopted the Gregorian calendar in 1873.
- Russia switched in 1918, after the revolution. That’s why Russia’s “October Revolution” of 1917 is remembered in November.
- Greece was the last European country, in 1923.
Many Orthodox churches still use the Julian calendar for religious festivals, which is why Orthodox Christmas falls on 7 January: the Julian 25 December is now 13 days behind.
The seven-day week
The week isn’t tied to the Sun or the Moon at all. The seven-day cycle goes back to ancient Babylon and to the Jewish Sabbath, and the Romans named the days after the seven objects they could see moving in the sky: the Sun, the Moon and five planets. In English, four of those names were swapped for Norse and Germanic gods:
| Day | Named after |
|---|---|
| Sunday | The Sun |
| Monday | The Moon |
| Tuesday | Tiw, a god of war (in Latin, Mars) |
| Wednesday | Woden, or Odin (in Latin, Mercury) |
| Thursday | Thor, god of thunder (in Latin, Jupiter) |
| Friday | Frigg, wife of Odin (in Latin, Venus) |
| Saturday | Saturn |
You can still hear the Latin names in Spanish and French: martes and mardi for Tuesday, from Mars.
Why some holidays move
Some holidays are fixed to a date, like Christmas on 25 December. Others move, for one of three reasons.
Rules about weekdays
Many public holidays are set to a weekday rather than a date: US Thanksgiving is the fourth Thursday in November, and Labor Day the first Monday in September. A 1968 US law moved several holidays to Mondays to create more three-day weekends. In the UK, most bank holidays fall on Mondays.
When a fixed-date holiday lands on a weekend, it’s often observed on another day. In the US, a Saturday holiday is usually observed on the Friday before, and a Sunday one on the Monday after. In the UK, it moves to the next weekday that isn’t already a holiday.
Easter and the Moon
Easter is set by the Moon. Since the Council of Nicaea in AD 325, it has been the first Sunday after the first full moon on or after 21 March, using the church’s tables rather than the actual Moon. That means Western Easter can fall anywhere from 22 March to 25 April. Many other Christian festivals are counted from Easter, such as Ash Wednesday (46 days before) and Pentecost (49 days after).
Orthodox churches use the same rule with the Julian calendar, so Orthodox Easter is often a week or more later, and only sometimes falls on the same day.
Other calendars
Several religious and cultural calendars follow the Moon, so their holidays land on different Gregorian dates each year:
- The Islamic calendar is purely lunar: 12 months of 29 or 30 days, making a year of about 354 days. Each holiday comes about 11 days earlier each year, cycling through all the seasons in about 33 years. Ramadan and Eid move this way, and a month traditionally begins when the new crescent moon is sighted, so the exact date can vary by a day between communities.
- The Hebrew calendar follows the Moon but adds a leap month seven times every 19 years to stay in step with the seasons. Passover is always in spring and Hanukkah in late November or December, though the exact date moves. Jewish holidays begin at sundown the evening before.
- Lunar New Year, or Chinese New Year, falls on a new moon between 21 January and 20 February.
- Diwali, the Hindu festival of lights, falls on a new moon in October or November.
Reusing an old calendar
Because the Gregorian calendar has only 14 possible layouts (a year can start on any of 7 days, and be a leap year or not), every calendar repeats. 2027 starts on a Friday and isn’t a leap year, so its dates fall on the same weekdays as 2021, and again in 2038. The moving holidays won’t match, though.
Or skip the old one: the printable calendar makes a fresh one for any year, with the holidays already worked out, including 2027.