I have come to learn that the Roman Catholic Church on the Greek Island of Syros celebrates Easter at the same time as the Eastern Orthodox Churches in general.
Who made the decision?
What authority can move the celebration days of Easter in the Catholic Church?
I can not find a source that says who permitted the Catholic Church on the Island of Syros to celebrate Easter on the same day as the Orthodox Churches. But understanding the integrate nature of the Church, it would be given to Catholic Church in Syros through the form of an indult from Rome, probably through the Congregation for Divine Worship and the Discipline of the Sacraments and with the approval of the Sovereign Pontiff, the Pope.
The Catholic Community of Syros Greece, Cyclades: The island of Syros hosts a vast Catholic community, which reaches almost half of the island's population. There is also an equal quantity of Catholic and Orthodox churches. The most impressive Catholic church is the church of Agios Georgios, on the highest point of Ano Syros village, with the Centre of Historical Studies of Catholic Bishopric on the back side of the church.
The equality between the Catholic and Orthodox people is an interesting fact, since Orthodoxy is actually the main dogma in Greece. This shows the influence of the Venetians, who conquered the island in the medieval times and occupied it for many centuries. During this period, Syros became a bastion of Catholicism and, in fact, it is known as the Island of the Pope.
Even when the Ottomans dominated the island, the Catholic influence remained and Syros came under the protection of France and the Vatican. Moreover, the French had ensured from the Turks some privileges for the Catholic community of Syros. Many residents became Catholic that period of time and Catholic monasteries were founded, such as the Capuchin Monastery and the Monastery of Jesuits in Ano Syros, that stand till today.
Later on, when Greece was deliberated from the Turks and the Greek State was established, the catholic population of Syros was Hellenized and changed their Latin names to Greek, for example Russo to Roussos, Vuccino to Voutsinos and Vacondio to Vakondios.
The integration to the Greek State did not provoke any problems and there was a good relationship between the old Roman Catholic residents of Syros and the refugees from Asia Minor, who arrived on the island in 1921 and were mostly Greek Orthodox. Therefore, the coexistence between both communities doesn't affect the peace and tranquility of the island. They have learned to go along with each other for centuries. - Syros Catholic Community