HISTORY OF LEAP YEAR DAY
A Year Is Not 365 Days
The earth's year is 365.24219 days long. That's how long it takes the Earth to make one cycle around the sun, and through the seasons. Until Julius Caesar proclaimed every fourth year a Leap Year, calendars were a mess. If you lived to be 90 years old, your birthday would have drifted by three weeks.
Now the calendar is pretty orderly, but a small minority of people don't get a birthday during most years. How small a minority??? Your chance of being born on Leap Year Day is one out of 1461 -- about 684 out of a million people are born on Leap Year Day -- or about 5 million people on the planet. If you are born on Leap Year Day, you can join the ranks of The Honored.
We can thank these guys:
45 BC: Emperor Julius Caesar invents the Julian Calendar and orders the calendars to add an extra 24 hours to February 24th every four years so that the seasons will come at the same time every year. At that time, February was the last month of the year, and the last five days of the year were feasting time, so February 24th was made a 48 hour day, once every four years. Hence the name "bisextile day" (a second day, six days before the new year). This was not a perfect solution, because a year is not quite 365.25 days long, the seasons will still eventually run behind, but not during a lifetime.
1582: Pope Gregory XIII, invents the Gregorian calendar. He makes a number of changes to the calendar, and corrects the calculation for Leap Day, to eliminate the slight seasonal drift in the Julian calendar. First, the end of the year was moved from February 28th to December 31st. A number of days were skipped to accomodate all the extra Leap Days that had occured since Caesar's time, and he creates a new calculation for Leap Day: three leap days are omitted every 400 years. If the year is divisible by 100 (i.e. a century year), then it is a Leap Year if the year is divisible by 400. (So 2000 was a Leap Year, but 1900 was not.)