In some countries, where the majority of Christians are of an Eastern Orthodox or Oriental Orthodox Church, (esp. in the Middle East), the Catholic Church permits Easter and its dependent movable feasts to be celebrated alongside her sister Orthodox Churches, which use the Julian calendar — at least for the calculation of Easter. Feasts of the sanctoral cycle — i.e. feasts with a fixed date — continue to follow the Gregorian calendar, however.
Today all Western countries — and almost all others — use the Gregorian calendar as their civil calendar. The Roman Catholic Church uses the Gregorian calendar for both calculating the date of Easter and the fixed feasts of the year (sanctoral cycle). Similarly, the Finnish Orthodox Church and several Eastern Catholic Churches (e.g. the Syriac Maronite Church of Antioch, the Melkite Greek-Catholic Church) use the Gregorian calendar for both the fixed and movable cycles of their respective liturgical calendars.
Most Eastern Orthodox Churches (e.g. the Ecumenical Patriarchate of Constantinople, the Church of Greece, the Greek Orthodox Church of Antioch, the Greek Orthodox Patriarchate of Alexandria and All Africa, the Romanian Orthodox Church) calculate the date of Easter and its dependent (movable) feasts according to the Julian calendar.
While these Eastern Orthodox Churches (e.g. Greek, Antiochian, Alexandrian, Romanian) may appear to use the Gregorian calendar for their fixed feasts (or sanctoral cycle), they in fact use the Revised Julian calendar (or Milanković calendar). This is despite the fact that they use the Julian calendar for dating Easter and its dependent feasts (as noted above). Dates in the Revised Julian calendar will be the same as those in the Gregorian calendar until 2800. It is for this reason, that the Eastern Orthodox Churches are thought to use the Gregorian calendar for their fixed feasts.
The Russian Orthodox Church and Mount Athos, and one or two others, continue to use the Julian calendar for all feasts, fixed and movable.