Despite the fact that they're really orbiting an instantaneous point in space that, uh, whatever Red said, the Earth orbits the sun. And it takes one year for it to happen. Sort of.
Actually, it takes roughly 365.2422 days for it to happen. So if you have a year that's always 365 days long, and do that for, oh say, 120 years, then the year would be starting a month earlier. Kinda makes a mess of things for your great-grandchildren's farmers.
So we added leap years. Roughly every four years, we add an extra day to the year. So far so good, except one day every four averages out to .25 days a year, and each year has an extra .2422 days. So we don't add a leap year for years divisible by 100, (like 1700, 1800, 1900), but we do for years divisible by 400 (like 1600, 2000, 2400). Which doesn't work out exactly to .2422, but it gets pretty close. All to get the calendar to work.