Constantine, a pre-Christian pagan, despised the Jews and rejected their 7th day sabbath. He was a sun worshiper. Sun-day was their holy day. This and Easter, etc are examples of how he corrupted the bishoprics with paganism. Thus, to this day, Christianity, to lesser and greater degrees, has been corrupted, particularly the RCC.
Easter, as has been pointed out, already existed. All the Council did was to agree on a way of reckoning the date.
And Sunday had already been adopted as a day of worship by the early Christians, and obviously this had nothing to do with paganism, but was because it was the "Lord's Day", the day of the resurrection; observation of the Sabbath being thought unnecessary or even heretical, since it was part of the Jewish ceremonial law, and was therefore considered unnecessary to salvation, like circumcision and keeping kosher.
So in St. Ignatius'
Epistle to the Magnesians (early second century, long before Constantine got involved) he writes:
If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again by Him and by His death whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher? And therefore He whom they rightly waited for, having come, raised them from the dead.
Let us not, therefore, be insensible to His kindness. For were He to reward us according to our works, we should cease to be. Therefore, having become His disciples, let us learn to live according to the principles of Christianity. For whosoever is called by any other name besides this, is not of God. Lay aside, therefore, the evil, the old, the sour leaven, and be changed into the new leaven, which is Jesus Christ. Be salted in Him, lest any one among you should be corrupted, since by your savour you shall be convicted. It is absurd to profess Christ Jesus, and to Judaize. For Christianity did not embrace Judaism, but Judaism Christianity, that so every tongue which believes might be gathered together to God.
Similarly Justin Martyr, writing in the mid second century in his
First Apology describes Sunday worship:
And on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things. Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen [...] Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Savior on the same day rose from the dead.
Whether or not the early Christians were right in adopting Sunday worship is a question I leave up to you, but it is clear that they had done so
before Constantine could have had anything to do with it, what with him being born in 272.
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