Leap year

A leap year is a year in the Gregorian calendar with 366 days instead of 365 &mdash; and unless you've been living on a planet with exact orbital resonance and some sort of self-adapting time system, you should be well aware of this. It is done because people with mad math skills and OCD must have everything line-up exactly. So if the Earth orbits the Sun in over 365.25 days, then dagnabbit, the calendar must reflect it! So the aforementioned people with the mad math skills and OCD add an extra day to the month of February, which is already really short, to make up for this extra 0.25 of a day.

Leap years are sort of misnamed because nothing is actually "leaped over" &mdash; in fact it could be argued that all the other years are leap years because they "leap over" February 29.

Calculating
Calculating when a leap year occurs is more Byzantine than it looks on the surface. A leap year occurs in any year evenly divisible by four, unless that year is also evenly divisible by 100. If a year is divisible by 100, then it's not a leap year unless it is evenly divisible by 400.

This results in 2000 being a leap year, but not 1900 or 2100.

If the year were exactly 365.25 days long, then making every fourth year a leap year would match exactly with the length of the year. This would mean that the seasons would slowly advance by a quarter of a day (6 hours) in every ordinary year, and then in the fourth year, that would be made up for by the leap year. But the year is actually slightly less than that: on the average, it is something like 365.2422 days long. That means that the seasons advance about 5 hours and 49 minutes each ordinary year, and then in the fourth year, when it is a leap year, it is overcompensated for by moving back an additional 24 hours, which sets the seasons back by about 44 minutes compared to their timing four years before. Over the course of a century, that overcompensation is multiplied 25 times: 25 times 44 minutes equals about 18 hours, almost a day after 100 years, so we can make up for that overcompensation by omitting one leap year in a century. That, in turn, would be about 24-18 = 6 hours too much, or 1 day in every 400 years. This results in the final rule for the Gregorian calendar: each fourth year is a leap year except every 100 years, which are leap years only every 400 years. The Gregorian calendar would be a perfect fit to the length of the year if the year were exactly 365.2425 days long.

There are more complicated algorithms which make for a better long-term match, but isn't it complicated enough as it is?

Why?
The Gregorian calendar, and its predecessor the Julian calendar, are solar calendars and thus use the Earth's orbit around the Sun as its base. As it turns out, Earth takes a little less than 365.25 days to orbit the Sun. To compensate for the almost quarter day, the leap year was invented by Roman astronomer Sosigenes in the year 46 BCE for then Roman dictator Julius Caesar. This leap year didn't have the rule regarding years divisible by 100, thus all years divisible by 4 were leap years. Julius Caesar then added extra days to 46 BCE to bring the seasons inline. This brought 46 BCE in at 445 days, the longest year on record. The resulting calendar is called the Julian Calendar.

The Gregorian calendar came about because the Julian calendar overcompensated and seasons started to drift &mdash; can't have the spring equinox in June, that's unnatural! To restore Easter to its proper place, Pope Gregory XIII decreed that ten days would be dropped from October 1582 CE. The calendar went from October 4 to October 15. This change didn't take effect immediately all over the world, making some odd discrepancies in calculating time across multiple years. By the 18th century, most places had adopted the Gregorian calendar, the most notable exception being Russia.

Historical evidence suggests that this was not an attempt to screw-over people unlucky enough to be born on February 29, who can't get drunk until they're 68.

Superstition
Because the 29th of February is an anomaly superstitions have developed round it, most suggest that the day is a lucky day and according to one website anything that is begun on a leap day will certainly succeed. Certainly? Now that is a bit strong. By contrast in Scotland some people think it unlucky to marry on leap day and in Greece some couples will not plan a marriage for that day. Further the couple will not celebrate their wedding anniversary on the correct day more often than every four years, since 25, 50 and 75 do not divide by four the couple cannot celebrate their important silver wedding, gold wedding, or diamond wedding on the correct day. It should surprise no one that superstitions developed in different areas sometimes contradict each other.