Easter, which celebrates Jesus Christ's resurrection from the dead, is Christianity's most important holiday. It has been called a moveable feast because it doesn't fall on a set date every year, as most holidays do. Instead, Christian churches in the West celebrate Easter on the first Sunday following the full moon after the vernal equinox on March 21. Therefore, Easter is observed anywhere between March 22 and April 25 every year. Orthodox Christians use the Julian calendar to calculate when Easter will occur and typically celebrate the holiday a week or two after the Western churches, which follow the Gregorian calendar.