Every four years, we have an extra day in the month of February. Why?
In normal (non-leap) years, there are 365 days in the year. But it actually takes 365 days and about 6 hours (more exactly, 365.24219 rotations of the Earth about it's axis) to complete a full rotation of Earth about the Sun (in other words, a full year). That ~quarter of a day adds up! If we don't account for it, eventually our calendar gets way out of line with the seasons over hundreds of years. To keep this from happening, we add a full day every four years, which keeps us on track with the actual time it takes us to travel about the Sun.