This animation shows why leap years are necessary. Without them, December would eventually drift into summer.

• Leap Day is February 29. A simple animation shows the purpose of this bonus day.
• Leap years realign our calendars with the equinoxes and solstices to keep up with changing seasons.
• Without leap years, December would drift into summer in 400 years.

Leap year happens once every four years for a good reason: Without it, December would eventually drift into summer.