Calendar

The calendar on this page, and on almost every wall, phone and computer in the world, is the Gregorian calendar. It is a solar calendar: its job is to keep the dates in step with the seasons, so that the March equinox stays around March 20 century after century. It does that with 365-day years and a leap day, February 29, added under a precise rule.

The leap-year rule

A year is a leap year if it is divisible by 4, except century years, which are leap years only if they are divisible by 400. So 2024 and 2028 are leap years, 1900 and 2100 are not, and 2000 was. That gives 97 leap days every 400 years and an average year of 365.2425 days. The US Naval Observatory notes that this is within about half a minute of the tropical year, the cycle of the seasons, so the calendar drifts by only about a day in 3,300 years.

Why calendars repeat

Four hundred Gregorian years contain exactly 146,097 days, which is exactly 20,871 weeks. The whole calendar, weekdays included, therefore repeats every 400 years: January 1, 2426 will fall on the same weekday as January 1, 2026. Within that cycle there are only 14 possible year calendars, one for each weekday that January 1 can fall on, in a common and in a leap year. That is why an old calendar can often be reused: 2026 has the same calendar as 2009, 2015, 2037 and 2043.

Month lengths

The old knuckle rule still works: make two fists and count the months across the knuckles and the gaps between them. Months that land on a knuckle have 31 days; months in a gap have 30, except February. July and August both land on knuckles, which is why they are both 31 days long.

Sunday or Monday first?

Nothing in the calendar itself says which day starts the week. The international standard for dates, ISO 8601, counts Monday as day 1 and Sunday as day 7, and most European calendars are printed that way. In the United States the printed week traditionally starts on Sunday, and in Egypt, Iran, Iraq, Jordan, Kuwait and several other countries on Saturday. Unicode’s locale data (CLDR) records each country’s custom and describes it as the first day of a calendar page view, which is not necessarily the first working day. Choose the one you are used to with “Week starts”; we remember it on this device.

Week numbers

Week numbers are common in Europe for planning and in business everywhere. ISO 8601 weeks start on Monday, and week 1 is the week that contains the first Thursday of the year, or equivalently January 4. The result is that every week belongs to exactly one year, but the days around New Year can belong to the neighbouring year: January 1, 2027 is a Friday in week 53 of 2026. The US convention starts weeks on Sunday and calls the week that contains January 1 week 1, so the two numbers often differ by one. The week number page explains both and shows the current week.

Days, dates and the year

Every date also has a position in the year, from day 1 on January 1 to day 365 (or 366) on December 31. In logistics and the military this ordinal day is often called a “Julian date”, written YYDDD; astronomers use Julian Day for a different thing, a continuous count of days since 4713 BC. The day of the year page and the Julian date page keep the two apart.

From the Julian to the Gregorian calendar

The Gregorian calendar replaced the Julian calendar, which had a leap year every fourth year without exception and had drifted against the seasons. In the first changeover, in 1582, Thursday, October 4 was followed by Friday, October 15. Great Britain and its colonies switched in 1752, when Wednesday, September 2 was followed by Thursday, September 14, eleven days later; the same Act moved the start of the legal year from March 25 to January 1. Russia changed in 1918, Greece in 1923 and Turkey in 1926. From March 1900 to March 2100 the old Julian calendar runs 13 days behind, so December 25 in the Julian calendar falls on January 7 in ours.

Holidays on this calendar

Holidays come from each government’s own published list: the US Office of Personnel Management, GOV.UK, the Canadian, Australian, Irish, New Zealand, South African, Singapore, Hong Kong, Japanese, French and Spanish authorities. We show only dates those sources publish for 2026–2028, and for 2025, 2029 and 2030 only holidays whose legal rule fixes the date (such as “the first Monday in September”), marked as computed. Holidays that depend on an announcement or a lunar calendar are never guessed. Public holidays lists each country with its source.