Three out of every 4 years, we observe 28 days during the month of February. Every 4 years, however, we add on a 29th day, or a "leap day". These years are known as leap years, and not only does 2012 happen to be a leap year, but Wednesday happens to be a leap day!
Traditionally, we think of our year as having 365 days. One year is the time it takes the earth to make one revolution on its orbit around the sun. But 365 days is just an approximation, with the actual time being 365.24 days, which is equal to 365 days plus 5 hours and 48 minutes. Rounding that to about 6 hours, it equals one fourth of a day. So the actual revolution is closer to 365 and 1/4 days.
So what does this mean? That means that after 4 years of "rounding" down, we have lost a whole day's worth of time that has been unaccounted for. Now, in the short term, it is negligible. What does it matter if it is May 24 when it is supposed to be May 25? But multiplying this error over time would make a BIG, noticeable difference! After about 400 years, our calendar would be off by close to 100 days. That's over 3 months!
That means, for example, our calendar would say July, but the position of the earth with respect to the sun would be where it typically is during October, so we would be experiencing October weather. That would make our job as meteorologists a bit more challenging!