The leap year rule
A year in the Gregorian calendar is a leap year if it is divisible by 4, except that century years are not leap years unless they are also divisible by 400. Running the rule on a few examples: 2024 divides by 4 and is not a century year, so it is a leap year. 2026 does not divide by 4, so it is a common year with 365 days. 1900 divides by 100 but not by 400, so despite dividing by 4 it was a common year. 2000 divides by 400, so it was a leap year, which is why many people who lived through it never saw the exception in action.
The reason for the rule is that a tropical year, one full orbit of the sun, is about 365.2422 days rather than a neat 365. Adding a day every four years overcorrects slightly, to 365.25 days. Skipping three leap days every 400 years brings the average calendar year to 365.2425 days, which drifts from the true year by only about one day in 3,200 years.
February 29 and its side effects
In a leap year February has 29 days and the year has 366, which is 52 weeks and 2 days. That extra day shifts weekdays: in a common year a given date moves forward one weekday from one year to the next, but across a February 29 it moves forward two. It also means a leap year has 53 of some weekday, and that the ISO week count can reach 53 when January 1 lands on a Wednesday in a leap year.
Roughly one person in 1,461 is born on February 29. For legal purposes such as turning 18 or 21, most US states treat the birthday as occurring on March 1 in common years, while software and medical records commonly use February 28. Payroll, interest and rent calculations that divide by 365 are slightly off in a leap year, which is why many loan contracts specify a 360-day or actual/actual day count.
Listing leap years and planning ahead
The range list is useful for scheduling anything on a four-year cycle. Between 2000 and 2050 there are 13 leap years: 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044 and 2048. The next century exception is 2100, which will not be a leap year, followed by 2200 and 2300; 2400 will be. Summer Olympics and US presidential election years have historically coincided with leap years, although the Olympic schedule shifted in 2021.