During the discussions that followed the Aleppo proposal for a common date for Pascha by a committee of the World Council of Churches, the consensus of opinion of the Orthodox Church is that one of the rules that was laid down at Nicaea I was that the Christian Pascha must fall following the Jewish Passover.
As I have argued above, if by "Jewish Passover" is meant "the 15th of Nisan in the Rabbinic calendar", then the consensus in wrong. The modern Rabbinic calendar did not even exist at the time of the Council, so the Julian Paschalion cannot have any built-in mathematical dependence on it.
Unfortunately, the minutes of the 1st Ecumenical Council have been lost. However, from other sources, we can determine its conclusions on the date of Pascha. The controversy was between the Churches in Asia Minor (modern Turkey) which celebrated Pascha on the day of the Jewish Passover regardless of the day of the week and Rome that celebrated Pascha on the Sunday after the Jewish Passover.
I'll believe in these hypothetical "minutes of Nicea" when you can produce them. I agree, though that we have enough evidence to make a good guess at what the council decided. And what it decided is almost nothing like what you describe.
Eusebius states the issue succinctly in the 5th chapter of his Life of Constantine
Some claimed that it was fit to follow the custom of the Jews; others claimed that the season [καιρος] ought to be observed accurately [ακριβη].
There is nothing here about weekday vs. Sunday observance. It is about accuracy. The "custom of the Jews" was considered unsuitable by some because it did not always "observe" the "season" accurately, not because it failed to place the festival on Sunday.
What this "custom of the Jews" was, and in what way it was inaccurate, is revealed by a number of sources. The 21st chapter of the Syriac Didascalia
Study to complete your vigil in the midst of [the Jewish days of] Unleavened Bread.
But the Didascalia
makes clear that Easter is always to be on Sunday. So it was not a question of Sunday observance, but of who should decide when the days of Unleavened Bread should fall.
We know that in that period, some Jewish communities set their Unleavened Bread before the Spring equinox. The Chronicon Paschale
quotes Peter, bishop of Alexandria in the early 4th century, as stating "the men [i.e. Jewish folk] of the present day celebrate [Passover] before the equinox." Post-conciliar sources, such as the Sardica Paschal Table and the anonymous homily of 387, indicate that some Jewish communities continued in the 4th century to place their 14th of Nisan, and hence the days of Unleavened Bread, before the equinox in most years. Those who followed the practice described in the Didascalia
and held their Easter within the Jewish days of Unleavened Bread would then sometimes have celebrated before the equinox. This was the "inaccuracy" in the "custom of the Jews", the "extraordinary mistake" complained of by Anatolius of Laodicea: celebrating Easter at the end of winter instead of in the springtime, the "season of joy" according to Eusebius:
When the sun traverses the first segment of its path [i.e. crosses the spring equinox], the moon with its fullness of light acts in parallel and restores the course of the night to the brightness of day...at this time the fields of the countryside are pregnant with seeds, and the trees teem with fruit and delight in Gods gifts, and yield to farmers the blessing of recompense for their labors.--De Solemnitate Paschali 2. Translation by Mark Delcogliano.
The place where this custom was followed can also be known, and it was not Asia minor; it was Syria. The Didascalia
is a Syrian work. The list of regions that, at the time of the Council, already follow the practice approved at Nicea is found in the 19th chapter of Eusebius's Life of Constantine:
[T]he city of Rome, and in Africa; throughout Italy, and in Egypt, in Spain, the Gauls, Britain, Libya, and the whole of Greece; in the dioceses of Asia and Pontus, and in Cilicia.
The province of Asia already follows the approved custom. The holdout region is Syria, which is not listed.
According to Nicaea I, Pascha must follow the Jewish Passover, which was celebrated on the 14 of Nissan, which was the day of the first full moon following the Spring Equinox. Since the adoption of the Gregorian Calendar, the Western Easter can fall on or before the Jewish Passover which is a violation of the decision of the First Ecumenical Council.
These two statements contradict each other. If the "Jewish Passover" that Easter must "follow" is "the first full moon following the Spring Equinox", than the Gregorian calendar never sets Easter before the "Jewish Passover" defined in this way. Gregorian Easter is, to a good approximation, the first Sunday after the first full moon after the Spring equinox. (The only time in the near future when this will not be the case will be in 2019, when there is an astronomical full moon shortly after the equinox that the Gregorian approximation misses.) But if the "Jewish Passover" is something that the Gregorian Easter can fall "before", then it must be something other than the Spring full moon, such as the 15th of Nisan in the Rabbinic calendar, which in about 3 years of every 19 comes at the time of the second full moon after the equinox. So which is it? Is it the first full moon of Spring, or the Rabbinic Matzoth, that is the "Passover" that Easter must be "after"?
We also know that the council entrusted Alexandria not Rome with determining the date of Pascha.
We can reasonably conclude that the council did no such thing. St. Athanasius's behavior in the years following the council is completely inconsistent with any such claim of privilege. The first bishop of Alexandria to make this bizarre claim of privilege seems to have been Cyril. The council, in fact, seems to have left all details of the computation to be worked out in practice by consultation among the bishops of the major cities. This is what the record shows St. Athanasius actually doing.
When Rome abandoned the Pascallion from Alexandria, it violated the standards set by the First Ecumenical Council
If "the standard set by the First Ecumenical Council" is to set Easter to the Sunday after the first full moon of Spring, then it is the Gregorian paschalion that meets this standard almost always, while the Julian paschalion meets it almost never.