Why do we have a leap year every four years?

A leap year is necessary because the Earth's orbital period around the Sun, known as a tropical year, is not an integer number of days. While a standard calendar year consists of 365 days, the actual time taken for one complete revolution is approximately 365.2422 days [2]. This fractional difference of roughly 0.25 days (or six hours) per year accumulates over time. If left uncorrected, the calendar would drift away from the astronomical seasons by about 24 days every century. To synchronize the calendar with the Earth's orbit and maintain seasonal consistency, an extra day is added every four years to February, bringing the average year length closer to the solar reality [2]. This adjustment ensures that events like the spring equinox remain relatively fixed on the same calendar date.

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > Sidereal Period > p. 260