This year is leap year, which means you will probably be reading this on February 29. This date comes around every four years in general, although we did not have one in 2000. The need for a leap year is due to the timing of the earth’s orbit around the sun. Here are some resources that explain them in more detail.
Marshall Shepherd of UGA posted an explanation in his blog at Forbes: http://www.forbes.com/sites/marshallshepherd/2016/01/04/2016-is-a-leap-year-why-do-we-have-them/#1c778599783b
Slate has a video explaining why we don’t have a leap day every four years here.
Vox.com also has a short video explanation at http://www.vox.com/2016/2/27/11121716/why-leap-year.