Today, February 29th 2012, is leap day! Every four years an extra day is added to the calendar in February to make up for the fact that one full year is actually ~365.25 days. But why is it defined this way and what does astronomy have to do with it?
Throughout history, many changes to our calendar have been made so that the seasons and solstices occur on roughly the same dates every year. Since the 16th century we have been using the Gregorian calendar system, which defines one year to be 365days, and one leap year to be 366 days. Every four years we have a leap year, except for years which are divisible by 100 and not divisible by 400. So, for example, the year 2000 was a leap year (divisible by 100 and 400) but the year 2100 will not be a leap year, because it's not evenly divisible by 400. If you do the math, this results in the average number of days in a Gregorian year to be 365.2425 days. This coincides with amount of time it takes the Earth to go around the sun once (~365.2425 days). This makes sense, but Earth's orbit does not stay in the exact same place in space year after year. This slight shift in Earth's orbit is called precession. This results in a tropical year (the time it takes to go from the exact time of the winter solstice one year to the next) to occur on a 365.24219 day schedule. So in general thing line up nicely, but actually, we are overestimating by a tiny amount. If we want the seasons to line up correctly, we will have to make an additional one day correction every ~26,0000 years. This also doesn't include other astronomical changes to the Earth that occur on even longer timescales, but during your lifetime you shouldn't notice any change between seasons and the dates they occur on.
