How long is a year? A calendar year is the amount of time that it takes the earth to move in one complete orbit (nearly a circle, but not quite) around the sun. A day on the other hand, is one complete rotation of the earth around it's own center.
The ancient Greeks were the first to realize that a year is not made up of a whole (1,2,3,...365...) number of days. Hipparchus, a great astronomer, mathematician and scientist of the day calculated (with amazing accuracy!) that a year consists of 365.246 days. That is to say that the Earth spins around itself like a top 365.246 times every time that it traces its ellipse through space around the sun.
Noting this fact, Julius Caeser established a system of time (now known as the 'Julian' calendar) which considered the year to be made of 365.25 days. If we wanted to make every year the exact same length, we would have two options. First, we could ignore the extra .25 days. The problem with this method is that the system of days and months would shift by roughly 6 hours per year. After 100 years, January 1st would come to be at the same time in the Earth's orbit of the sun that January 25th used to be! After 1000 years, New Year's Day would be near the end of the summer, on September 8th! Clearly unacceptable.
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