A day is the amount of time it takes for a planet, like the Earth to rotate, or turn around, once on it's axis. The axis is the imaginary line upon which the Earth spins, loosely from the North Pole through the center of the Earth to the South Pole. It takes Earth approximately 24 hours to rotate once on it's axis. It is important to note the period of rotation for all the planets is not the same, some take longer to otate, while others may be faster. For example, Mercury takes almost 59 Earth days to rotate once on it's axis, while it takes 88 Earth days to revolve, or go around, the sun. That means a day on Mercury is almost as long as a year!