Well lthere are 365 days in a year.
This is supposed to be how long it takes for the earth to rotate around the sun.
Really it take 365 days and 6 hours.
So every four years they add one more day to make up for those six hours (6hrsx4=24hrs=1day), that way everything matches up with the way its supposed to be.
It's added to February because , before February was the last month in the year in ancient times.
Another little known fact is that, though we use the Roman calendar... it is actually the Gregorian calendar as it is they who added the Leap Year day to correct Caesar's mistake.
Just saw that on the news this morning a few minutes ago.