Every four years (with some extremely rare exceptions), Feb. 29 is added to our calendars. It's called a leap year or a leap day.
Why is there a leap year?
It takes about 365.25 days (more specifically 365.2422 days) for Earth to make one revolution around the sun, according to NASA. To make up for the extra time, an additional day is added to the calendar at the end of February every four years.
So in short, that's a leap year.
To break it down even further, the length of a year is based on how long it takes a planet to revolve around its sun. Since Earth's days don't perfectly line up with the Earth's orbit, an extra day is added to keep time and seasons in order.
Without a leap day, the dates of annual events, like equinoxes and solstices, would slowly shift to later in the year. NASA points out that after only a century without leap days, summer wouldn't start until mid-July.
Dr. James O'Donoghue, a planetary scientist for the Japan Aerospace Exploration Agency, posted two helpful animations online which further illustrates why Leap days exist.
O'Donoghue's animation shows just how quick our seasons would be disrupted. It reveals that without leap years, December would drift into summer in 400 years.
The leap year solution gets a little trickier because the extra .2422 day is about six hours. NASA explained if the orbit was exactly 365 days and six hours, adding a single day every four years wouldn't be a problem. However, Earth takes just a little less time than that. So, rounding up and adding an extra day every four years adds about 45 minutes, or about three full days every 400 years.
Stay with me.
To correct the extra 45 minutes, years that are divisible by 100 do not have leap days unless they also can be divided by 400.
For example, the year 2000 was a leap year (it was divisible by 100 and 400), however, the years 2100, 2200, and 2300 will not have a leap year because they aren't divisible by 400.
Scientists really planned it all out.