Today is February 29, a date that only happens every fourth year. Why do we add a day to the year's shortest month every fourth year? Simple: the year isn't exactly 365 days long, but rather 365 ¼, or pretty close to it. It's that extra ¼ that brought about the Leap Year.
The ancients knew the length of the year was roughly 365 days. Unfortunately, those extra few hours were discounted. So, to set the scene, with the year actually being 365 ¼ days long, every 4 years, the calendar would become 1 day out of sync with real astronomical time. By the end of a century, the calendar would be almost a month off (1 day off every 4 years for 100 years makes for 25 days off of actual time by the end of 100 years). However, none of the ancient civilizations did anything to correct this, but instead merely put up with a calendar that was frequently out of sync with the seasons.
Enter Julius Caesar.
After he won the Roman Civil War and became dictator, Julius Caesar finally decided to do something about the out of sync calendar problem. Knowing that the year was actually 365 ¼ days long, Caesar decreed that there would be an extra day added to every 4th year to eliminate the problem of the inaccurate calendar, hence the start of the leap year.