schrikkeljahr

Schrikkeljaar, Dutch for leap year, is a calendar year containing 366 days instead of the usual 365. The extra day is added to February, resulting in February 29 as the leap day.

In the Gregorian calendar, leap years occur if the year is divisible by 4, with two exceptions. Century years (such as 1800, 1900, 2100) are not leap years unless they are divisible by 400 (such as 1600 and 2000). Consequently, 1996, 2004, and 2020 are leap years, while 1900 was not and 2000 was.

Historically, Julius Caesar introduced a 4-year cycle with a leap day in the Julian calendar. The Gregorian reform of 1582 refined the rule to correct drift against the tropical year, which is about 365.2422 days. The adjustment keeps the vernal equinox near March 21 and maintains alignment of the calendar with the seasons over time.

Over a 400-year cycle the calendar contains 97 leap years, totaling 146,097 days. Most countries using the Gregorian calendar apply this rule consistently, while other calendars and historical periods used different systems for intercalation. The leap year concept remains a means to maintain long-term calendar accuracy with the solar year.