Easter is celebrated on different dates by Roman Catholics and Protestants (5 April) and Orthodox Christians (a week later) due to the adoption of the Gregorian calendar by the former and the continued use of the Julian calendar by the latter. The Council of Nicaea established that Easter should be celebrated on the first Sunday after the first full moon of spring, but the differing calendars lead to the discrepancy. Orthodox Easter includes unique traditions such as a midnight mass and egg games.