Every four years February gets an extra day. The reason is that the Earth refuses to orbit the Sun in a whole number of days, and a calendar that ignores the remainder slowly slides away from the seasons.
One orbit of the Sun takes about 365.2422 days. A calendar year of 365 days is therefore just under a quarter of a day too short.
That sounds harmless. It is not, given enough time. The error accumulates at almost exactly one day every four years, one week every 29 years, and a full month every 124 years. Left uncorrected for a few centuries, midsummer would drift into spring and eventually into winter. Every festival tied to a season would come loose from it.
The leap day is the fix: add a day every fourth year and the calendar keeps pace with the orbit.
Most people know the first line of the rule and stop there. There are three:
So 1900 was not a leap year, while 2000 was. That is why anyone who lived through 2000 has never actually seen the exception in action, and why a fair amount of software written in the nineties got it wrong.
The reason for the extra clauses is that adding a day every four years is slightly too generous. Four years of the simple rule adds 365.25 days per year on average, but the real figure is 365.2422. The difference is small, about 11 minutes a year, and it adds up to three days every four centuries. Skipping three leap days out of every four century years removes exactly those three.
Julius Caesar introduced the four-year rule in 46 BC, and it stood for more than sixteen centuries. But 11 minutes a year is still an error, and by the 1500s the calendar had drifted about ten days. Easter, which is anchored to the spring equinox, was visibly out of place.
Pope Gregory XIII reformed it in 1582: ten days were deleted outright, and the century rule was added to stop the drift returning. Catholic Europe switched immediately. Protestant Europe, unwilling to take calendar instructions from Rome, took much longer.
This is where Sweden earns a footnote in the history of timekeeping.
Rather than deleting eleven days at once, Sweden decided in 1700 to phase them out gently, by skipping every leap day for forty years. The first step went to plan: 1700 had no 29 February. Then the Great Northern War arrived, attention went elsewhere, and the plan was simply forgotten. The leap days of 1704 and 1708 were observed as normal.
The result was a calendar one day ahead of the Julian one and ten days behind the Gregorian, matching nothing anyone else used. In 1712 Sweden gave up and decided to go back to the Julian calendar, which required putting the missing day back. February that year was given 30 days.
30 February 1712 existed in Sweden and nowhere else, ever. Sweden finally adopted the Gregorian calendar in 1753, the sensible way, by deleting seventeen days at once.
The extra day shifts weekdays by one, which is why a date rarely falls on the same weekday two years running. It also interacts with the ISO week system: because a leap year has 366 days, it is more likely to be one of the years containing 53 weeks rather than 52.
We go through how the weeks are counted in week numbers and ISO 8601.
Nollan.se shows the date, the week number and the name day for whatever day it happens to be, leap day included.
A year is a leap year if it divides evenly by 4, except century years, which must also divide by 400. So 2024 and 2028 are leap years, 1900 was not, and 2000 was. The next ones are 2028, 2032 and 2036.
Because adding a day every four years overshoots slightly. A solar year is about 365.2422 days, not 365.25, so the simple rule adds roughly three days too many every four centuries. Skipping three century leap days out of four corrects almost exactly that.
Most pick 28 February or 1 March in ordinary years, and there is no official rule. In Swedish law a person born on 29 February is treated as having their birthday on 28 February in non-leap years for age-related purposes.
Yes, in 1712, and no other country ever has. Sweden had begun a slow, gradual switch to the Gregorian calendar and then abandoned it halfway, ending up aligned with neither calendar. To get back to the Julian one, an extra day was added to February that year.
Almost. The Gregorian rule is off by roughly one day every 3200 years, which nobody has bothered to correct. Leap seconds are a separate matter entirely, and deal with the Earth's rotation rather than its orbit.