In Western Christianity, Easter always falls on a Sunday from March 22 to April 25 inclusive. The following day, Easter Monday, is a legal holiday in many countries with predominantly Christian traditions. In Eastern Christianity, Easter falls between April 4 and May 8 between 1900 and 2100 based on the Gregorian date.
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The Church of Rome used its own methods to determine Easter until the 6th 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 8th 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 use the original Julian calendar, their dates are not usually aligned in the present day.