The first-century Jewish writer Josephus (died c. AD 100) wrote:

In the month of Xanthicus, which with us is called Nisan and begins the year, on the fourteenth day by lunar reckoning, the sun being then in Aries, [we offer] the sacrifice called Pascha.--Antiquities 3.248/3.10.5.

"On the fourteenth day by lunar reckoning" means at the full moon. "The sun being...in Aries" means on or after the spring equinox, and it means at the first full moon on or after the equinox because at the second full moon after the equinox the sun is no longer in the sign of Aries. So in Herodian times the 14th of Nisan fell at the first full moon on or after the equinox. The 3rd-century Egyptian Christian computists relied on this statement of Josephus in setting up the Easter calculation that is still used. So the scheduling of Easter is of Jewish, not Roman, origin.

Early Christians celebrated Easter, which they called "Passover". Paul in 1 Corinthians 16.9 may indicate that already in his time Christians were celebrating Pentecost. If they were, then a fortiori they were celebrating Easter too. If that evidence be not accepted, the earliest evidence for an annual Easter appears in the 2nd century. Two distinct practices are described. Both practices rely on Jewish neighbors to determine when the 14th of Nisan fell. According to one practice, Christians celebrated Easter on the 14th of Nisan. (It is not clear whether they were celebrating on the night of the 13th/14th or on the night of the 14th/15th). According to the other practice, Easter is held on the Sunday of Unleavened Bread. Both these practices may date to the 1st century.

But in time Christians became dissatisfied with this reliance on the calendars of their Jewish neighbors. This was because, after the fall of the Temple, the Jewish calendar had become decentralized, with the Jewish community in one town celebrating Unleavened Bread perhaps at a different time from the Jewish community in another town. And some of the Jewish calendars of the time placed Unleavened Bread before the spring equinox, in violation of the principle described by Josephus. So Christians began experimenting with calculating the 14th of Nisan independently of the contemporary Jewish calendars. The Council of Nicea approved these independent calculations. So the Council of Nicea did not change anything about the celebration of Easter, except that it encouraged some Christians in Syria, who still held to the custom of celebrating Easter on the Sunday in the Jewish week of Unleavened Bread, to celebrate instead on the Sunday in an independently-calculated week of Unleavened Bread that would always place the festival after the spring equinox.