A leap year is a year containing an additional day added to the calendar. In the Gregorian calendar, which is the most widely used calendar in the world, leap years occur every four years. The additional day is added to the month of February, meaning February has 29 days in leap years instead of the usual 28.

The concept of leap years originates from the need to align the calendar with the Earth's revolution around the Sun. The duration of the Earth's orbit is not exactly 365 days but is closer to 365.2422 days. This means that the regular 365-day calendar would gradually become out of sync with the seasons, leading to discrepancies in dates and astronomical events over time.

The leap year system was introduced to compensate for this difference. By adding an extra day every four years, the calendar stays more closely aligned with the Earth's orbit and prevents the seasons from shifting.

However, there are some exceptions to the four-year leap year rule. The year 1900 was not a leap year, and neither will be the year 2100. This is because the Gregorian calendar follows a slightly more complex rule:

A year is a leap year if it is divisible by 4 but not by 100, or if it is divisible by 400.

So, while every fourth year is generally a leap year, an exception is made for century years (years ending with "00"). Century years are not leap years unless they are also divisible by 400. This adjustment helps to further fine-tune the calendar's alignment with the Earth's orbit.

By considering these exceptions, the Gregorian calendar can maintain a more accurate synchronization between calendar dates and astronomical events over long periods. Leap years serve as a necessary adjustment to bridge the slight difference between the Earth's orbital period and the standard calendar year, ensuring that the calendar remains in harmony with the natural cycles of the seasons.