A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. In the Gregorian calendar, the most widely used calendar system today, a leap year has 366 days instead of the usual 365 days. This extra day is added as February 29th, making February 29 days long instead of its usual 28 days.

The concept of leap years was introduced because Earth's orbital period around the Sun (a tropical year) is approximately 365.2422 days. Without the occasional addition of a leap day, our calendar would gradually drift out of alignment with the seasons. Over time, this would cause significant discrepancies, with summer months eventually occurring in what we now consider winter months.

The Gregorian calendar uses three simple rules to determine if a year is a leap year. First, if the year is evenly divisible by 4, it is a potential leap year. Second, if the year is evenly divisible by 100, it is NOT a leap year, unless it also meets the third rule. Third, if the year is evenly divisible by 400, then it IS a leap year regardless of the second rule.

For example, the year 2024 is divisible by 4 and not by 100, so it's a leap year. The year 1900 was divisible by 4 and by 100 but not by 400, so it was not a leap year. However, the year 2000 was divisible by 4, by 100, and by 400, making it a leap year. This system ensures that our calendar remains accurate over centuries, with an average year length very close to the actual tropical year.