The thing I find odd is how so much is made over Julian vs. Gregorian (or modified versions), when significant differences existed or were debated from ancient times till now, like how the year they lived in was spoken of, which changed a bunch of times through the years; or in what year Jesus was born, which has also been disputed since ancient times. Obviously for purposes of festivals, fasts, readings, saints days, etc. the calendar has a utilitarian role... but it's not like all Christians in all places have
ever had the same calendar or time keeping scheme.
The part about a common Easter was originally not in a canon at Nicea, but in the "Synodal Letter" sent to Alexandria telling them about what had happened at the Council; the part about Easter said:
We further proclaim to you the good news of the agreement concerning the holy Easter, that this particular also has through your prayers been rightly settled; so that all our brethren in the East who formerly followed the custom of the Jews are henceforth to celebrate the said most sacred feast of Easter at the same time with the Romans and yourselves and all those who have observed Easter from the beginning.
Easter is a separate issue from the calendar in general though, and had been in dispute for a couple centuries at that point. They wanted to get all Christians celebrating Easter at the same time, which is understandable, but it seems from what happened at various times, such as with the dispute between Rome and Asia Minor in the early 2nd century (as described by Eusebius in
Ecclesiastical History, 5.23-24), that unity and peace was considered more important than perfect uniformity:
Synods and assemblies of bishops were held on this account, and all, with one consent, through mutual correspondence drew up an ecclesiastical decree, that the mystery of the resurrection of the Lord should be celebrated on no other but the Lord's day, and that we should observe the close of the paschal fast on this day only. There is still extant a writing of those who were then assembled in Palestine, over whom Theophilus, bishop of Cæsarea, and Narcissus, bishop of Jerusalem, presided. And there is also another writing extant of those who were assembled at Rome to consider the same question, which bears the name of Bishop Victor; also of the bishops in Pontus over whom Palmas, as the oldest, presided; and of the parishes in Gaul of which Irenæus was bishop, and of those in Osrhoëne and the cities there; and a personal letter of Bacchylus, bishop of the church at Corinth, and of a great many others, who uttered the same opinion and judgment, and cast the same vote...
But the bishops of Asia, led by Polycrates, decided to hold to the old custom handed down to them. He himself, in a letter which he addressed to Victor and the church of Rome, set forth in the following words the tradition which had come down to him...
Thereupon Victor, who presided over the church at Rome, immediately attempted to cut off from the common unity the parishes of all Asia, with the churches that agreed with them, as heterodox; and he wrote letters and declared all the brethren there wholly excommunicate.
But this did not please all the bishops. And they besought him to consider the things of peace, and of neighborly unity and love. Words of theirs are extant, sharply rebuking Victor.
Among them was [St.] Irenæus, who, sending letters in the name of the brethren in Gaul over whom he presided, maintained that the mystery of the resurrection of the Lord should be observed only on the Lord's day. He fittingly admonishes Victor that he should not cut off whole churches of God which observed the tradition of an ancient custom and after many other words he proceeds as follows:
For the controversy is not only concerning the day, but also concerning the very manner of the fast. For some think that they should fast one day, others two, yet others more; some, moreover, count their day as consisting of forty hours day and night.
And this variety in its observance has not originated in our time; but long before in that of our ancestors. It is likely that they did not hold to strict accuracy, and thus formed a custom for their posterity according to their own simplicity and peculiar mode. Yet all of these lived none the less in peace, and we also live in peace with one another; and the disagreement in regard to the fast confirms the agreement in the faith.
He adds to this the following account, which I may properly insert:
Among these were the presbyters before Soter, who presided over the church which you now rule. We mean Anicetus, and Pius, and Hyginus, and Telesphorus, and Xystus. They neither observed it themselves, nor did they permit those after them to do so. And yet though not observing it, they were none the less at peace with those who came to them from the parishes in which it was observed; although this observance was more opposed to those who did not observe it.
But none were ever cast out on account of this form; but the presbyters before you who did not observe it, sent the eucharist to those of other parishes who observed it.
And when the blessed Polycarp was at Rome in the time of Anicetus, and they disagreed a little about certain other things, they immediately made peace with one another, not caring to quarrel over this matter. For neither could Anicetus persuade Polycarp not to observe what he had always observed with John the disciple of our Lord, and the other apostles with whom he had associated; neither could Polycarp persuade Anicetus to observe it as he said that he ought to follow the customs of the presbyters that had preceded him.
But though matters were in this shape, they communed together, and Anicetus conceded the administration of the eucharist in the church to Polycarp, manifestly as a mark of respect. And they parted from each other in peace, both those who observed, and those who did not, maintaining the peace of the whole church.
Thus Irenæus, who truly was well named, became a peacemaker in this matter, exhorting and negotiating in this way in behalf of the peace of the churches. And he conferred by letter about this mooted question, not only with Victor, but also with most of the other rulers of the churches.
St. Irenaeus speaks of those who differed following "the tradition of an ancient custom," but in regards to certain elements there
was no uniform ancient custom, but different customs existed in different places, depending on where in the world that you were. The old calendarist arguments rely heavily on excluding areas outside the Mediterranean world, and also significant lengths of time, during and at which a much larger variety of practices can be seen. Orthodox often speak harshly about Latinizing, but there was a good deal of attempted Byzantinizing of the Eastern Local Churches going on as well.
In the end, peace and unity was preferred over trying to force changes by viewing differences as schisms or making declarations about heterodoxy. St. Ireneaus said: "the disagreement in regard to the fast confirms the agreement in the faith." It'd be good for everyone to be on the "same page" regarding Easter and the calendar, but there being disagreement doesn't mean that the disagreement has to lead to division.
