Why is the Eastern Orthodox celebrated at a different time from the rest of the Christian World?

Answer 1

Eastern Orthodox Churches and along with with most other Churches, which are not derived from the Roman Catholic Church, the computation is done in accordance to the same rules for over 1000 years. Roman Church has changed the calendar to Gregorian along with shift to the Easter Sunday estimation.

The main reason for the Orthodox Churches to keep the Julian Calendar for Easter, even when the fix-day celebrations are according to Gregorian Calendar, is that Resurection Day must always be after the Jewish Passover. With Western/Gregorian style computations, it comes not only on the same date, but often quite earlier, which is unacceptable for the Orthodox Church.