The origin of the leap year as it’s known today dates back to the 16th century and was adopted to resolve a lag created by the Gregorian calendar, the standard calendar for most of the world for the past 500 years. Every four years, with some exceptions, an extra day gets added to the month of February, resulting in 366 days instead of the normal 365. It’s a small revision that keeps the days of the year on track with earth’s orbit around the sun. The next leap day will take place Monday, February 29, 2016.

The Gregorian calendar, also called the Western or Christian calendar, was introduced in 1582 and named for its creator, Pope Gregory XIII. It was adopted first in Italy, Poland, Portugal and Spain and was established to realign with the equinoxes – the times of year when the day and night are equal lengths – and solstices – the year’s longest and shortest days. The Gregorian calendar succeeded the Julian calendar, the timetable instituted in Rome in the 1st century B.C. under the reign of Julius Caesar.

The reason for the leap year comes down to the earth’s revolutions around the sun. The exact time it takes the Earth to make a full orbit, known as a solar year, is about 365.242199 days, or 365 days, 5 hours, 48 minutes and 46 seconds, according to Time and Date. The extra time is about the equivalent of an extra quarter of a day, meaning that every four years, the calendar has an extra day to make up for.

The first civilization to recognize the leap year was the ancient Egyptians, according to Info Please. The Romans later adopted the practice into their calendar and designated Feb. 29 the official leap day.

But it’s not a perfect system. If the leap day rule was simply observed every four years, it would add an additional day to the calendar every 128 years. Therefore, the rule was tweaked a bit to exclude years divisible by 100, unless also divisible by 400. The change has allowed the calendar to skip a leap year every few hundred years.

In the U.S., about 200,000 people were born on a leap day at any given time. Worldwide, that number jumps to about 5 million. “The law of averages means your chance of being born on February 29 are one out of 1,461,” Peter Brouwer, co-founder of the Honor Society of Leap Year Day Babies in Keizer, Oregon, told Mail Online in 2012.