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A calendar year with an additional day or month is added to maintain synchronization with the astronomical or seasonal year. This correction helps correct the drift between a civilization's dating system and the physical properties of the Solar System, as astronomical events and seasons do not repeat in a whole number of days.

An astronomical year lasts slightly less than 365 days, with the Julian calendar having three common years of 365 days and a leap year of 366 days. The Gregorian calendar adjusts for this error by adding an extra leap day in each year that is a multiple of 4. The Hebrew calendar adds Adar Aleph seven times every 19 years to keep its calendar year from drifting through the seasons. The term "leap year" comes from the extra day in the week, causing a fixed date to advance two days.

The choice of February as the month to add a leap day can be traced back to Julius Caesar's calendar reforms in ancient Rome. Caesar created the Julian calendar, which included a leap year to align the calendar year with the solar year.
Even after the Julian calendar evolved into the Gregorian calendar in 1582, the tradition of adding a leap day to February continued.

What if we didn't have leap years?
If we didn't have a leap year and didn't take into account the extra time between calendars, the seasons would start to drift apart.
Over the course of about 700 years, for example, summer in the Northern Hemisphere will begin in December instead of June.

What is the frequency of leap years?
Most people think that leap years occur once every four years, but this is not always the case.
This is because the difference between a sidereal year (the exact number of days, hours, minutes and seconds) and calendar years over a four-year period is not exactly 24 hours. It is actually 23.262222 hours.
Therefore, this difference must also be accounted for.
The rule is that a year is considered a leap year if it can be divided by four to produce an integer.
There is one exception, which is years that can be divided by 100, as they are not considered leap years.
However, if a year produces an integer when divided by 400, it is still considered a
leap year.
It sounds complicated, let's explain with the following examples:
The year 2000 was a leap year, because the number can be divided by both 4 and 400.
But the years 1700, 1800 and 1900 can be divided by 4 but not by 400, so they were not leap years.
The next time a leap year will be skipped is the year 2100.