I found the letter of Emperor St Constantine (from ccel.org):
"On the Keeping of Easter.
From the Letter of the Emperor to all those not present at the Council.
(Found in Eusebius, Vita Const., Lib. iii., 18–20.)
When the question relative to the sacred festival of Easter arose, it was universally thought that it would be convenient that all should keep the feast on one day; for what could be more beautiful and more desirable, than to see this festival, through which we receive the hope of immortality, celebrated by all with one accord, and in the same manner? It was declared to be particularly unworthy for this, the holiest of all festivals, to follow the custom [the calculation] of the Jews, who had soiled their hands with the most fearful of crimes, and whose minds were blinded. In rejecting their custom, we may transmit to our descendants the legitimate mode of celebrating Easter, which we have observed from the time of the Saviour’s Passion to the present day [according to the day of the week]. We ought not, therefore, to have anything in common with the Jews, for the Saviour has shown us another way; our worship follows a more legitimate and more convenient course (the order of the days of the week); and consequently, in unanimously adopting this mode, we desire, dearest brethren, to separate ourselves from the detestable company of the Jews, for it is truly shameful for us to hear them boast that without their direction we could not keep this feast. How can they be in the right, they who, after the death of the Saviour, have no longer been led by reason but by wild violence, as their delusion may urge them? They do not possess the truth in this Easter question; for, in their blindness and repugnance to all improvements, they frequently celebrate two passovers in the same year. We could not imitate those who are openly in error. How, then, could we follow these Jews, who are most certainly blinded by error? for to celebrate the passover twice in one year is totally inadmissible. But even if this were not so, it would still be your duty not to tarnish your soul by communications with such wicked people [the Jews]. Besides, consider well, that in such an important matter, and on a subject of such great solemnity, there ought not to be any division. Our Saviour has left us only one festal day of our redemption, that is to say, of his holy passion, and he desired [to establish] only one Catholic Church. Think, then, how unseemly it is, that on the same day some should be fasting whilst others are seated at a banquet; and that after Easter, some should be rejoicing at feasts, whilst others are still observing a strict fast. For this reason, a Divine Providence wills that this custom should be rectified and regulated in a uniform way; and everyone, I hope, will agree upon this point. As, on the one hand, it is our duty not to have anything in common with the murderers of our Lord; and as, on the other, the custom now followed by the Churches of the West, of the South, and of the North, and by some of those of the East, is the most acceptable, it has appeared good to all; and I have been guarantee for your consent, that you would accept it with joy, as it is followed at Rome, in Africa, in all Italy, Egypt, Spain, Gaul, Britain, Libya, in all Achaia, and in the dioceses of Asia, of Pontus, and Cilicia. You should consider not only that the number of churches in these provinces make a majority, but also that it is right to demand what our reason approves, and that we should have nothing in common with the Jews. To sum up in few words: By the unanimous judgment of all, it has been decided that the most holy festival of Easter should be everywhere celebrated on one and the same day, and it is not seemly that in so holy a thing there should be any division. As this is the state of the case, accept joyfully the divine favour, and this truly divine command; for all which takes place in assemblies of the bishops ought to be regarded as proceeding from the will of God. Make known to your brethren what has been decreed, keep this most holy day according to the prescribed mode; we can thus celebrate this holy Easter day at the same time, if it is granted me, as I desire, to unite myself with you; we can rejoice together, seeing that the divine power has made use of our instrumentality for destroying the evil designs of the devil, and thus causing faith, peace, and unity to flourish amongst us. May God graciously protect you, my beloved brethren."
I have to say that despite the fact there is no canon of the First Ecumenical Council concerning the calendar, this document makes clear that the Council made a firm and binding decision concerning the celebration of the feast on the same date. This means that a Council may make firm decisions that are not necessarily set down in canons, but are nevertheless considered binding on all the Church. This in turn means that when the Council of 1583 affirmed that the Seven Councils made it a law to follow the patristic Paschalion and Menologion, we have to take that affirmation seriously, and not dismiss it because there is no canon specifying a penalty for disobedience. By impugning the authority of the 1583 Council, you are more or less claiming to have greater authority than a Council in determining what the Church has taught concerning the calendar. As an Orthodox Christian, I find myself unable to accept your authority as greater.