For all the details about why we have a leap day (almost) every four years, visit the Leap Year Internet site at http://www.urc.ukans.edu/News/96N/FebNews/Feb20/leapyear.html. For the history of the calendars, including the Gregorian calendar, which we use and which mandates the insertion of leap days, go to http://astro.nmsu.edu/~lhuber/leaphist.html.
If you're looking for something to do on that special day, visit the "leap year capital of the world", the town of Anthony on the Texas-New Mexico border for the quadrennial Worldwide Leap Year Festival, or, if you're short of cash for the air fair, visit the web site at http://www.leapyearcapital.org/.
Leap seconds are a whole different matter, and they are added by international agreement to compensate for small variations in the earth's rotation. Go to http://tycho.usno.navy.mil/leapsec.html for the US Navy's take on leap seconds.