A leap year, also known as an intercalary year, is the one containing an extra day, normally February 29th, although this varies in certain calendars. In the Gregorian Calendar, which is used by most of the world, a leap year occurs every four years or 97 years out of every 400. This is done as a way to keep seasons, astronomical events, and time differences in sync. Without leap years, the Gregorian calendar would lose veracity in just over a hundred years, leading to time differences between day and night, and moving the equinox early. The Gregorian method is by far the most accurate way to creating leap years. By adding a 29th day in all years divisible by 4 (except for years ending in -00), the Gregorian calendar will fall behind only one day every 8000 years.

Other calendars don’t do such a good job of creating leap years. Examples of that are the Julian and the Coptic calendar, which create leap years by simply adding an extra day every four years, regardless of any other consideration. This method, which was in common use until a few centuries ago, would push the calendar a day forward every 130 years.

The Hebrew calendar, which is lunisolar, adds an extra month rather than an extra day, to form a leap year. This is done by using a complicated rule, creating seven leap years every nineteen. Because the first day of the Hebrew year cannot fall on Wednesday, Friday, or Sunday, the rules adjust depending on the year, so it’s hard to estimate when a leap year will come. Other calendars that add leap days include the Indian National Calendar (extra day in February/March), the Hindu Calendar (an extra month, added when two dark moons occur), and the Chinese Calendar (an extra month added using a complicated lunar system).

There are several traditions and superstitions based on leap years. An old Irish legend says that women are only allowed to propose marriage on leap years; men must take the initiative any other time. Other traditions considered February 19th an unlucky day in which new enterprises, trips, and partnerships should not be started. People who are born on leap years usually celebrate their birthdays on 28th February, which is usually considered the legal birthday of people born on a leap year.