The feast of Easter is linked to the Jewish Passover and Feast of Unleavened Bread, as Christians believe that the crucifixion and resurrection of Jesus occurred at the time of those observances.
As early as Pope Sixtus I, some Christians had set Easter to a Sunday in the lunar month of Nisan. To determine which lunar month was to be designated as Nisan, Christians relied on the Jewish community. By the later 3rd century some Christians began to express dissatisfaction with what they took to be the disorderly state of the Jewish calendar. They argued that contemporary Jews were identifying the wrong lunar month as the month of Nisan, choosing a month whose 14th day fell before the spring equinox.
Christians, these thinkers argued, should abandon the custom of relying on Jewish informants and instead do their own computations to determine which month should be styled Nisan, setting Easter within this independently computed, Christian Nisan, which would always locate the festival after the equinox. They justified this break with tradition by arguing that it was in fact the contemporary Jewish calendar that had broken with tradition by ignoring the equinox, and that in former times the 14th of Nisan had never preceded the equinox. Others felt that the customary practice of reliance on the Jewish calendar should continue, even if the Jewish computations were in error from a Christian point of view.
The controversy between those who argued for independent computations and those who argued for continued reliance on the Jewish calendar was formally resolved by the Council, which endorsed the independent procedure that had been in use for some time at Rome and Alexandria. Easter was henceforward to be a Sunday in a lunar month chosen according to Christian criteria—in effect, a Christian Nisan—not in the month of Nisan as defined by Jews. Those who argued for continued reliance on the Jewish calendar (called “protopaschites” by later historians) were urged to come around to the majority position. That they did not all immediately do so is revealed by the existence of sermons, canons, and tractswritten against the protopaschite practice in the later 4th century.
These two rules, independence of the Jewish calendar and worldwide uniformity, were the only rules for Easter explicitly laid down by the Council. No details for the computation were specified; these were worked out in practice, a process that took centuries and generated a number of controversies. (See also Computus and Reform of the date of Easter.) In particular, the Council did not decree that Easter must fall on Sunday. This was already the practice almost everywhere.
Nor did the Council decree that Easter must never coincide with Nisan 14 (the first Day of Unleavened Bread, now commonly called “Passover”) in the Hebrew calendar. By endorsing the move to independent computations, the Council had separated the Easter computation from all dependence, positive or negative, on the Jewish calendar. The “Zonaras proviso”, the claim that Easter must always follow Nisan 14 in the Hebrew calendar, was not formulated until after some centuries. By that time, the accumulation of errors in the Julian solar and lunar calendars had made it the de facto state of affairs that Julian Easter always followed Hebrew Nisan 14.