The Gregorian calendar, explained

A Roman calendar, corrected

The Gregorian calendar is a repair of the Julian calendar, introduced by Julius Caesar in 46 BCE. The Julian year has 365 days with a leap day every fourth year, for an average of exactly 365.25 days.

That is very close to the length of the solar year — about 365.2422 days, measured from one March equinox to the next — but not close enough. The Julian year runs roughly 11 minutes long, and those minutes add up to a full day about every 128 years. By the 1500s the calendar had drifted about ten days: the spring equinox, which the Church used to calculate the date of Easter, was arriving around 11 March instead of 21 March.

1582: ten days that never happened

Pope Gregory XIII issued the reform in a papal bull, Inter gravissimas, dated 24 February 1582. It did two things at once:

1. It removed the accumulated drift. In the countries that applied it immediately — Spain, Portugal, most of Italy and Poland–Lithuania — Thursday 4 October 1582 was followed by Friday 15 October.
2. It changed the leap-year rule so the drift would not return.

October 1582

MonTueWedThuFriSatSun 12341516171819202122232425262728293031

567891011121314

Notice that the days of the week were not touched: Thursday was still followed by Friday. Only the date numbers jumped.

The 4-100-400 leap-year rule

The Julian rule — a leap year every fourth year — adds too many leap days. The Gregorian rule removes three of them every 400 years:

1. Divisible by 4? No → Common · 365 days e.g. 2026 Yes ↓
2. Divisible by 100? No → Leap · 366 days e.g. 2028 Yes ↓
3. Divisible by 400? No → Common · 365 days e.g. 1900, 2100 Yes ↓
4. Yes → Leap · 366 days e.g. 2000

So 2000 was a leap year, but 1900 was not and 2100 will not be. Over 400 years there are 97 leap years instead of 100, giving an average year of 365.2425 days — only about 26 seconds longer than the solar year. The calendar drifts by roughly one day in three thousand years or so, depending on which astronomical year you compare it with.

A neat consequence: 400 Gregorian years contain exactly 146,097 days, which is precisely 20,871 weeks. The calendar therefore repeats itself, weekdays included, every 400 years.

Twelve uneven months

Gregorian months have 28 to 31 days, in a pattern that has no astronomical meaning — it is inherited from Rome.

Gregorian

1. 31 Jan
2. 28+ Feb
3. 31 Mar
4. 30 Apr
5. 31 May
6. 30 Jun
7. 31 Jul
8. 31 Aug
9. 30 Sep
10. 31 Oct
11. 30 Nov
12. 31 Dec

Where the names come from

• January — Janus, the two-faced god of doorways and beginnings.
• February — Februa, a Roman festival of purification.
• March — Mars, the god of war.
• April — uncertain; possibly from Latin aperire, “to open”.
• May and June — the goddesses Maia and Juno.
• July and August — renamed in honour of Julius Caesar and the emperor Augustus.
• September to December — simply “seventh” to “tenth” in Latin (septem, octo, novem, decem). The early Roman year began in March, so these really were the seventh to tenth months.

Why February is so short

February was the last month of the early Roman year and absorbed the leftover days, which is why it alone has 28. You may have heard that Augustus took a day from February to make his month as long as Julius Caesar’s. It’s a good story, but historians trace it to the 13th-century scholar Johannes de Sacrobosco; there is no ancient evidence for it.

A slow, uneven adoption

The reform was a papal decree, so Catholic countries adopted it first. Others followed over the next three and a half centuries — often by skipping more days, because the gap kept growing.

1. 1582

   Catholic Europe

   Spain, Portugal, most Italian states and Poland–Lithuania skip from 4 to 15 October.

2. 1752

   Great Britain and its colonies

   Under the Calendar (New Style) Act 1750, Wednesday 2 September 1752 is followed by Thursday 14 September — eleven days skipped. The legal year now starts on 1 January instead of 25 March.

3. 1873

   Japan

   Japan replaces its lunisolar calendar, jumping to 1 January 1873.

4. 1918

   Russia

   After the revolution, Russia skips thirteen days in February 1918.

5. 1923

   Greece

   Greece, among the last European states to switch, moves from 15 February straight to 1 March for civil purposes.

Several Orthodox churches still use the Julian calendar for religious feasts, which is why Orthodox Christmas falls on 7 January in the Gregorian calendar.

Year numbers, weeks and ISO dates

Gregorian years are numbered from the traditional year of Jesus’ birth, a scheme devised by the monk Dionysius Exiguus in the 6th century. There is no year zero: 1 BCE is followed directly by 1 CE.

The international standard ISO 8601 writes dates as year–month–day, for example 2026-11-01, and starts the week on Monday. That order is unambiguous, unlike 01/11/2026, which means 1 November in most of the world but January 11 in the United States. It is also why ShamsiDate shows Gregorian dates in ISO form by default.

The Gregorian calendar in Iran

In Persian, the Gregorian calendar is called میلادی (miladi, “of the birth”). Its month names reached Persian through French, which is why January is ژانویه (from janvier) and August is اوت (août). Iranians use it alongside the Shamsi calendar for travel, trade and anything international — which is exactly where a reliable converter helps. Try the converter or read about the differences between the two calendars.

Sources

1. Encyclopaedia Britannica, “The Gregorian calendar”. britannica.com
2. Calendar (New Style) Act 1750, 24 Geo. 2 c. 23. legislation.gov.uk
3. “List of adoption dates of the Gregorian calendar by country”, Wikipedia. en.wikipedia.org
4. Jean Meeus, Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998), chapter 7 on calendar arithmetic.