While we generally say a year has 365 days, it actually takes 365.2422 days for the Earth to orbit around the sun. Over time, that extra quarter day would add up, and every four years, our calendar would fall behind one day. After 120 years, we'd be off a month, and in 720 years, December would be in the summer.
Thus, the "Leap Day" to get the calendar back on track -- well, sort of. We still have to account for that .0078 days by skipping the Leap Day every 100 years, which we do on the '00' years. So 1800 and 1900 did not have leap days. BUT! It's still not quite exact, so every 400 years, we DO add back in the leap day. So 2000 had the leap day, but we won't have another on the '00' year until 2400.
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