Why do we have an extra day every 4 years? So we say we have 365 days in a year, but it actually takes 365.25 days for the Earth to orbit the sun. This is called a solar year. We add one day to our calendar every 4 years to make up for that quarter of a day extra. This is leap year. Leap year actually has 366 days in it, rather than the usual 365 days. Because we have subtracted 1/4 of a day for 3 years, we must add that extra day on to our calendar the 4th year!
Leap years are important. We want the calendar year to match the solar year. Subtracting 1/4 of a day may not seem like a lot (only 6 hours a year!) but over the years, the calendar would be way off. All those missing hours, over years and years, would end up being days, weeks, and even months. Imagine celebrating the 4th of July, with snow on the ground!
So in a nutshell, having leap day every 4 years prevents a drift of 24 days every century, and keeps our holidays and seasons on track.
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