The calendar has to follow the seasons
The cycle of seasons lasts about 365.2422 days, rather than exactly 365. If every calendar year had only 365 days, calendar dates would drift relative to the seasons. Adding an extra day in some years makes the average length more accurate.
The rule is more subtle than every four years
The Gregorian calendar adds February 29 in years divisible by four, with an exception for century years that are not divisible by 400. That means 2000 was a leap year, while 1900 was not and 2100 will not be one.
Why the century exception matters
Adding one day every four years would yield an average calendar year of 365.25 days, a little too long. Skipping three leap days across every 400-year cycle reduces the average to 365.2425 days, much closer to the seasonal year.
What to remember
Leap years are a practical adjustment to keep the months lined up with the seasons. The rule is a calendar convention, not a claim that Earth moves in identical orbits every year.
Read the original sources
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