What’s the point of a Leap Year?

Every so often, the shortest month of the year, February, is given one extra day, making it 29 days long.

2024 is one of those years, making this year a leap year, where, in addition to February having 29 days, the year itself has a total of 366 days. The event normally occurs once every four years, and many people aren’t aware of what exactly the extra day does aside from making the year longer.

Here’s everything you need to know about leap years and why they are important.

Each calendar year is typically 365 days long, with the idea that it takes that many days for the Earth to make one complete rotation around the sun. But, according to the National Air and Space Museum at the Smithsonian, that number is actually rounded down, which means it actually takes the Earth 365.242190 days to orbit the sun, or 365 days five hours 48 minutes and 56 seconds.

This leaves six hours of the year unaccounted for, and, if they remain that way, the seasons will drift. Although the more drastic effects wouldn’t happen for centuries, eventually if a leap year were to never happen, people in the United States would end up celebrating Christmas in June.

If an extra day is added every four years, then the calendar adjusts back to normal, as we round again up to six and create a 24-hour day.

But because we rounded, this makes the calendar longer by 44 minutes. To account for the difference, a leap year is not necessarily every four years. The rule is that if the year is divisible by 100 and not divisible by 400, the leap year is skipped. The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not.  The next time a leap year will be skipped is the year 2100.

The reason why the year is called a leap year is because normally a person’s birthday is moved one day over every year. For example, if it was on a Monday one year it would be on Tuesday the next year. But a leap year adds an extra day so the birthday would now “leap” over a day, so if your last birthday was on a Monday, it would now be on a Wednesday.

History offers some further explanations behind the tradition. The first Leap Day was introduced by Julius Caesar on the Julian Calendar, which was established in 45 BC. It came after Roman Emperor Augustus, who began his rule in 27 BC, is believed to have wanted his dedicated month to have the same number of days as the month named after Julius Caesar.

At the time, historians say the month of February had 30 days, while the month of August had 29. Meanwhile, the month of July had 31 days.

It has been theorised that the emperor decided to remove two days from February to be added onto August, making August a 31-day month and February a 28-day one.

Historians also think that February became the chosen month for Leap Day because the Romans didn’t much care for the month and saw it as a “leftover month that nobody really wanted to begin with”.