The Gregorian Calendar, and its predecessor the Julian Calendar, are not perfect. The day represents one full rotation of Terra, the Month (kind of) coincides with the lunar cycle, and the year is Terra's full revolution around Sol. Almost.

While the calendar shows 365 days, it actually takes approximately 365.25 days for Terra to revolve around Sol. But only approximately, so the saga of the Leap Day goes deeper. And once you peel back THAT layer, it goes deeper still. Famed astrophysicist and host of Star Talk Neil deGrasse Tyson explains the full 400 year scope of the Leap Day for National Geographic.