Leap Year

Women are allowed to propose to their significant others once every 4 years according to Irish folklore, and that day is coming up, ladies - February 29, to be exact.

What're the odds that a parent and child not only have the same birthday, but will only get to celebrate it every four years?  About 2 million to one, apparently.

Once every for years we have a February 29th. We call it a leap year, but why do we have to add this extra day to our calendars?