The Greek Orthodox Church
From the Apostle Paul in Corinth to the Ecumenical Patriarchate today.
The Greek Orthodox Church refers to the family of Orthodox Christian communities whose liturgical language is Greek and whose mother church is the Ecumenical Patriarchate of Constantinople. It includes the Church of Greece, the Orthodox Patriarchates of Alexandria, Antioch, and Jerusalem, the Church of Cyprus, and the Greek Orthodox Archdiocese of America.
The Ecumenical Patriarchate
Founded by the Apostle Andrew and elevated to primacy at the Council of Chalcedon (451), the Ecumenical Patriarchate holds a "primacy of honor" among the Orthodox churches. The Ecumenical Patriarch resides at the historic Phanar quarter of Constantinople (modern Istanbul).
Calendar
Most Greek Orthodox jurisdictions follow the Revised Julian calendar for fixed feasts (making, for example, Christmas fall on December 25 of the civil calendar) while still calculating Pascha by the older Julian paschalion. A minority (the "Old Calendarists") continue to use the Julian calendar for everything.
In America
The Greek Orthodox Archdiocese of America, headquartered in New York and led by an Archbishop under the Ecumenical Patriarchate, is the largest single Orthodox jurisdiction in the United States.