Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun.
While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up.
To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens.
Julius Caesar introduced Leap Day, with help from the Egyptians. During his time in Egypt, Caesar became convinced of the superiority of the Egyptian solar calendar, which featured 365 days and an occasional intercalary month which was inserted when astronomers observed the correct conditions in the stars.
Caesar and the philosopher Sosigenes of Alexandria made one important modification: instead of relying on the stars, they would simply add a day to every fourth year.
In keeping with the Roman tradition of messing with the length of February, that day would fall in the second month of the year—thus Leap Day was born. Caesar added two extra-long months to the year 46 BCE to make up for missed intercalations, and the Julian calendar took effect on January 1st, 45 BCE.
People born on Leap Day are called ‘Leaplings’