This extra day, which falls on February 29 and is called Leap Day or Leap Year Day, exists to keep the calendar year in synchrony with the astronomical year, or the time it takes Earth to make a full rotation around the sun. Because seasons and astronomical events aren’t uniform on a global scale, a calendar that has a set number of days—like our traditional 365-day year (which, crucially, is closer to 365.25 days year), also called a common year—would cause these events to drift over time.
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