Thank goodness for Julius Caesar, Pope Gregory XIII, and leap years to keep our calendars in order. According to the U.S. Naval Observatory in Washington, D.C., the nation’s timekeepers, if we didn’t have leap years our calendar would be totally scrambled and out of sync with the seasons.
The reason, the Earth doesn't orbit the sun in exactly 365 days. It actually takes 365.2425 days. Over time those extra digits add up to almost a full day, but not quite, every four years.
About 500 leap days have come and gone since the Roman ruler Caesar first added a leap day to the calendar every four years around 46 B.C., but it wasn’t perfect as the Julian calendar was adding too many days. By the 1500s Easter was about 10 days off and slipping back into winter.
The modern calendar used today dates back to 1582, when Pope Gregory XIII instituted the Gregorian calendar, with its first leap year of 1584.
Here is where the Gregorian calendar gets interesting. Under its rules, every year that is evenly divided by 4 is a leap year, except for turn of the century ones ending with a "00" such as 1900 or 2100, that aren't divisible by 400.
Without this adjustment the seasons would gradually slip out of adjustment with the calendar, but at a really slow rate of about eight days every 1,000 years.
It is interesting to note that, according to the Honor Society of Leap Year Babies, roughly 5 million people worldwide share a February 29th birth date, making them "leapsters" or "29ers, according to the Honor Society of Leap Year Day Babies.