The Early Christians
n“The earliest Christians celebrated the Lord's Passover at the same time as the Jews, during the night of the first full moon of the first month of spring (Nisan 14-15). By the middle of the 2nd century, most churches had transferred this celebration to the Sunday after the Jewish feast. But certain churches of Asia Minor clung to the older custom, for which they were denounced as ‘judaizing'. The first ecumenical Council of Nicaea in 325 decreed that all churches should observe the feast together on a Sunday”

- The Encyclopaedia Britannica, 15th edition, Macropaedia, Vol. 4, pp. 604-605, "Church Year".