Leap Day happens since as part of the Gregorian reform of the Calendar, an extra day was needed to be added since the solar year was really 365 and 1/4 days long. If a leap day wasn't added the seasons will be out of sync with the calendar. Every 4 years we have a leap year except in years that are divisible by 100. If the year is divisible by 400 then we have a leap year. So 2000 was one but 1900 wasn't. But Gregorian reforms aside, the one reason why leap days and years are of interest is because of a long standing tradition that a woman may propose marriage to a man on these years and on leap day itself, it was ILLEGAL for a man to refuse! A supposed 1288 law promulgated by Queen Margaret of Scotland made it illegal to refuse. The penalties were once so drastic. (Was it death by hanging and quartering? Not really!) A man had to pay 1 pound which was then a huge sum of money. Because the law was so harsh the Queen reduced the penalty to a kiss. The man has give a kiss to the ...
"It is surely harmful to souls to make it a heresy to believe what is proved" - Galileo