Happy Leap Day! It only happens every four years so that's a reason to celebrate, right?
Why do we have leap years? Caution math and science ahead.
One year equals one trip around the Sun, which we count as 365 days. It actually takes 365.2422 days to completely orbit the Sun. To account for the "lost" quarter of a day, we tack on an extra day to the month of February - Leap Day. Without a Leap Day every four years, our calendar would be off 24 days every 100 years.
Not all leap years are created equal. As stated before, a leap year occurs when the year is divisible by four, i.e. every four years. The start of a new century is where things get a little tricky. It is not a leap year if the year can be evenly divided by 100, unless it is also divisible by 400. It was not a leap year in 1900, but it was in 2000. The years 2100, 2200, and 2300 won't be leap years, but 2400 will be. Why did someone have to make it so difficult? Well, it comes down to math again. If we have a leap year every four years, then it would still give us about 11 minutes too much each year and our calendar would be 18 hours off within 100 years.