Easter is a moveable feast celebrated on the first Sunday after the vernal equinox full moon (i.e. on Sunday after the full moon of March).
These dates are calculated with the old Gregorian calendar, because it specifies the vernal equinox. To find out when Easter is celebrated according to the new calendar, we have to add 13 days.
The calculations show that the dates of the orthodox Easter range from April 4 to May 8. Moreover, the Orthodox Easter dates are calculated based on the vernal equinox and therefore under the old calendar both Old and New Calendarists celebrate Easter on the same day. In contrast, the Roman Catholics and Protestants calculate the dates of the vernal equinox according to the new calendar, so they usually celebrate Easter a week earlier than the Orthodox Church.
Easter happens to be celebrated at the same time by the Western and the Orthodox churches, when the first full moon after the vernal equinox is after March 28.
The Church of Greece accepted the new calendar on March 10 1924, following the Greek state that had already accepted it in 1923, but without moving Easter and the other movable feasts that still depend on the old calendar.
The calculation of the Easter date is a complex mathematical problem. Years ago, the date of Easter was calculated with the help of a number of tables, the famous Easter tables. Today the date of Easter (both Orthodox and Catholic) can be calculated through certain mathematic formulas.