Easter Sunday, the day Christians celebrate the resurrection of Jesus Christ, falls on a different day each year, and in a given year, may be celebrated on two different Sundays by Christians in the Western Church and those in the Eastern Orthodox Church. Easter falls on the first Sunday after the first full moon of spring, but the Western Church uses the Gregorian Calendar, and the Eastern Orthodox Church uses the Julian Calendar.
In the year 325, the Council of Nicaea established that the Church would hold Easter on the first Sunday after the first full moon occurring on or after the vernal equinox, to correspond with Biblical account of the crucifixion occurring around the Jewish Passover holiday. Even after the Schism of 1054, when the Roman Catholic and Orthodox Churches split, both churches determined the date of Easter the same way.
In 1582, Pope Gregory XIII instituted a new calendar for all Roman Catholic countries. By the 16th century, the spring equinox had shifted from March 21, because a year is actually 11 minutes longer than 365 days. The new Gregorian calendar remedied this discrepancy by instituting the leap year system and removing 13 days from the year on a one-time basis. The Eastern Orthodox Church continued to use the Julian calendar, instituted under the reign of Julius Caesar, to calculate Easter. In some years, such as 2004, 2007, 2010 and 2011, Easter falls on the same date in both calendars.