Once every four years, an extra day is added to the month of February. That year is considered a leap year, and February 29 is known as a leap day.
Leap years exist because it takes the Earth 365 days, five hours, 48 minutes and 45 seconds to orbit the sun -- almost an extra quarter of a day per year. If you count the quarter of a day up for each year, you get an extra day added every fourth year.
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If we didn't use leap years, our calendars would be off by approximately 25 days after 100 years, according to AccuWeather. It's important to note, though, that the method isn't perfect. Leap years are skipped on century years that are evenly divided by 100 (like 1900 and 2100). They're not, however, skipped on century years that are evenly divided by 400 (like 2000 and 2400).