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There is no edict related to the Council of Nicea (325 AD) which had anything to do with Easter or Sundays.

There was a 321 edict forbidding work (except agricultural work, which was the work of the great majority of folks) on Sundays.

Further Constantine was present only for the first day of the Council of Nicea and long gone by the time:

Separation of Easter from the Jewish Passover

After the June 19 settlement of the most important topic, the question of the date of the Christian Passover (Easter) was brought up. This feast is linked to the Jewish Passover, as the crucifixion and resurrection of Jesus occurred during that festival. By the year 300, most Churches had adopted the Western style of celebrating the feast on the Sunday after the Passover, placing the emphasis on the resurrection, which occurred on a Sunday. Others however celebrated the feast on the 14th of the Jewish month Nisan, the date of the crucifixion according to the Bible’s Hebrew calendar (Leviticus 23:5,John 19:14). Hence this group was called Quartodecimans, which is derived from the Latin for 14. The Eastern Churches of Syria, Cilicia, and Mesopotamia determined the date of Christian Passover in relation to the 14th day of Nisan, in the Bible’s Hebrew calendar. Alexandria and Rome, however, followed a different calculation, attributed to Pope Soter, so that Christian Passover would never coincide with the Jewish observance and decided in favour of celebrating on the first Sunday after the first full moon following the vernal equinox, independently of the Bible’s Hebrew calendar.
According to Duchesne,[15] who founds his conclusions:
on the conciliar letter to the Alexandrians preserved in Theodoret;[16]
on the circular letter of Constantine to the bishops after the council;[17]
on Athanasius;[18]
Epiphanius of Salamis wrote in the mid-4th Century, “… the emperor … convened a council of 318 bishops … in the city of Nicea. … They passed certain ecclesiastical canons at the council besides, and at the same time decreed in regard to the Passover that there must be one unanimous concord on the celebration of God’s holy and supremely excellent day. For it was variously observed by people…”[19]
The council assumed the task of regulating these differences, in part because some dioceses were determined not to have Christian Passover correspond with the Jewish calendar. “The feast of the resurrection was thenceforth required to be celebrated everywhere on a Sunday, and never on the day of the Jewish passover, but always after the fourteenth of Nisan, on the Sunday after the first vernal full moon. The leading motive for this regulation was opposition to Judaism, which had dishonored the passover by the crucifixion of the Lord.”[20] Constantine wrote that: “… it appeared an unworthy thing that in the celebration of this most holy feast we should follow the practice of the Jews, who have impiously defiled their hands with enormous sin, and are, therefore, deservedly afflicted with blindness of soul. … Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Saviour a different way.”[21] Theodoret recorded the Emperor as saying: “It was, in the first place, declared improper to follow the custom of the Jews in the celebration of this holy festival, because, their hands having been stained with crime, the minds of these wretched men are necessarily blinded. … Let us, then, have nothing in common with the Jews, who are our adversaries. … avoiding all contact with that evil way. … who, after having compassed the death of the Lord, being out of their minds, are guided not by sound reason, but by an unrestrained passion, wherever their innate madness carries them. … a people so utterly depraved. … Therefore, this irregularity must be corrected, in order that we may no more have any thing in common with those parricides and the murderers of our Lord. … no single point in common with the perjury of the Jews.”[22]
The Council of Nicaea, however, did not declare the Alexandrian or Roman calculations as normative. Instead, the council gave the Bishop of Alexandria the privilege of announcing annually the date of Christian Passover to the Roman curia. Although the synod undertook the regulation of the dating of Christian Passover, it contented itself with communicating its decision to the different dioceses, instead of establishing a canon. There was subsequent conflict over this very matter. See also Computus and Reform of the date of Easter.

http://en.wikipedia.org/wiki/First_Council_of_Nicaea