Leap Days Explained: Leap days keep our calendar in alignment with Earth’s revolutions around the Sun. It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun. This is called a tropical year, and it starts on the March equinox.
However, the Gregorian calendar has only 365 days in a year. If we didn’t add a leap day on February 29 almost every four years, each calendar year would begin about 6 hours earlier in relation to Earth’s revolution around the Sun.
As a consequence, our time reckoning would slowly drift apart from the tropical year and get increasingly out of sync with the seasons. With a deviation of approximately 6 hours per year, the seasons would shift by about 24 calendar days within 100 years.
Allow this to happen for a while, and Northern Hemisphere dwellers will be celebrating Christmas in the middle of summer in a matter of a few centuries.
Leap days fix that error by giving Earth the additional time it needs to complete a full circle around the Sun.
