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Why leap years exist at all
Earth actually takes about 365.2422 days to orbit the sun, not a clean 365. If calendars ignored that extra roughly quarter-day, the calendar would drift out of sync with the seasons by about 24 days every century — solstices and equinoxes would gradually creep across the calendar until December fell in what used to be summer. Adding a 366th day, February 29, every four years absorbs almost all of that drift. The Gregorian calendar (adopted in 1582 and still in use today) refined this further with the century-year exception described below, tightening the average calendar year to 365.2425 days — within about 26 seconds of the real solar year.