This year, 2024, is a leap year. February 29 is known as "Leap Day". But what lies behind this rare extra day in the calendar?

Everyone knows that the Earth takes 365 days to orbit the sun. Well, that's not entirely true. The Earth actually takes 365 days, five hours, forty-eight minutes, and forty-six seconds to complete its orbit around the sun.

So, approximately every four years an extra day occurs and is represented in the calendar.

The Jet Propulsion Laboratory at the California Institute of Technology said that the extra day is used to ensure that the months remain aligned with annual events such as equinoxes and solstices.

Meanwhile, NASA confirms that over time, our seasons would become out of sync without the leap day. Each calendar year adds about six hours over 365 days.

However, a leap day is not added every four years. That would lengthen the calendar by 44 minutes, according to experts at the National Air and Space Museum in Washington, DC. Over time, this would mean summer would arrive in November, said Younis Khan, a physics teacher at the University of Alabama, Birmingham.

Therefore, a leap day is added approximately every four years, but not in years that include 1700, 1800, and 1900. There was a leap day in 2000 because it was divisible by 100 and 400. Over the next 500 years, there will be leap days, but there will be no leap day in 2100, 2200, 2300, and 2500.

There will be leap days in 2028, 2032, and 2036.

Khan said that without a leap day, farmers might have difficulty planting in the correct season. "Christmas would be in summer. There would be no snow. He said, "There won't be a Christmas feeling."

You might wonder who came up with the idea of the leap day. Experts say that the calendar change evolved over time.

Ancient civilizations planned their years based on the moon or the sun. By the time Julius Caesar controlled the Roman Empire, things became more complicated. People wrote calendars but the seasons were "moving". For example, the weather in July was not always the same. So, at that time, they added months to the calendar. But this didn't really solve the problem.

The Romans solved the problem in 46 BC. The length of the year became 365 and a quarter days. Therefore, an extra day was added every four years. The Romans came close, but the year is actually 365 plus 0.242 days, said Nick Ex, an astronomy professor at the University of North Carolina at Chapel Hill.

There were multiple calendars used by Western societies. The Athenian and Julian calendars have been used for thousands of years. Thomas Palermo, a classics professor at the University of Texas at Austin, said the Athenian calendar faced problems when holidays did not fall in the correct seasons.

Over time, the current calendar also changed due to holidays. The leader of the Roman Catholic Church, Gregory XIII, fixed the calendar again in the late sixteenth century. He wanted to ensure that the Christian Easter occurred in early spring. So Gregory and his advisors performed difficult calculations to arrive at something very close to the calendar we use today.

Ex, the astronomy professor, said: "If the solar year were 365.25 perfect, we wouldn't worry about the difficult calculations involved."

After the calendar, the main focus of Leap Day is on those born on that day, February 29. It comes only once every four years (mostly).

It is the start of a conversation for many. It can be an administrative headache – some people must fill out official documents stating their birthday as either February 28 or March 1. Some computer systems have not yet been updated to include February 29 as an option.

Only about 5 million out of 8 billion people worldwide were born on February 29. Shelley Dean from Seattle, Washington is one of them. She said that growing up, she held regular birthday parties every year, but every four years, she held a special party.

She is now 23 years old, but she hasn't had many "special" birthdays. So she is planning for this year. She missed celebrating with her family four years ago because she was going to school in New York. So, she will celebrate on Thursday her special day for the first time in eight years with her family.

She said, "It's a very great day."