At the
First Council of Nicaea in 325 it was decided that Easter would be celebrated on the same Sunday throughout the Church, but no method was specified by the Council. Instead, the matter was referred to Alexandria. The practice of this city was to celebrate Easter on the first Sunday after the earliest fourteenth day of a lunar month that occurred on or after
March 21. During the Middle Ages this practice was more succinctly phrased as
Easter is observed on the Sunday after the first full moon on or after the day of the vernal equinox. The Church of Rome used its own methods to determine Easter until the
sixth century, when it may have adopted the Alexandrian method as converted into the
Julian calendar by
Dionysius Exiguus (certain proof of this does not exist until the
ninth century). Most churches in the
British Isles used a late
third century Roman method to determine Easter until they adopted the Alexandrian method at the
Synod of Whitby in
664. Churches in western continental Europe used a late Roman method until the late
eighth century during the reign of
Charlemagne, when they finally adopted the Alexandrian method. Since western churches now use the
Gregorian calendar to calculate the date and Eastern Orthodox churches the original
Julian calendar, their dates are not usually aligned in the present day.