In the leap years on the centuries divided by 400. It's also estimated that we might need an extra day on top of the current system in about 8000 years time, but might need to wait and see about that. Read about it somewhere on Wikipedia recently, doing some research on leap days for an article, so if you look into leap days or the Gregorian calendar system you'll find the answer.
Actually Phil Plait explains it all - all! - in
his blog today. Go read!