A leap year is a year that contains an extra day, February 29, to keep the calendar year synchronized with the astronomical year. The reason for adding an extra day is to compensate for the fact that Earth's orbit around the sun takes approximately 365.25 days.
In the Gregorian calendar, which is the most widely used calendar system today, a leap year occurs:
1. Every four years: If a year is evenly divisible by 4, it is a leap year. For example, the years 2012, 2016, and 2020 were leap years.
2. Exception to the rule: Years that are divisible by 100 are not leap years unless they are also divisible by 400. For instance, the year **** was not a leap year, but the year 2000 was.
This system helps to keep the calendar year in close alignment with the tropical year, which is the time it takes for the Earth to complete one orbit around the sun, approximately 365.2422 days. The leap year adjustment ensures that the calendar year and the astronomical year remain relatively synchronized over time.