This is not an easy question to answer. There is much to take into account for the transformation of Sabbath to Sunday worship for Christianity. I will keep it to a brief minimal. History shows that after the fall of Jerusalem (~68 A.D.) Judean-Christian practices were suppressed and dissolved under the influences of the early Caesars permeating to an all time low. A new premise for Christians eventually opened up under the campaign of Emperor ‘Constantine the Great’ (~324 – 337 C.E.)
In his civil reign magistrates complied with legislated Sunday observance (~321 C.E.) in all of the Roman Empire.
New considerations followed for Christians by the established dual Church-State formation giving organization to a string of church councils. A formal gathering of bishops at Arles (~317 C.E.), Nicaea (325 C.E.) resolved the Easter debate. This paved the way for treatise and established Church canons at Laodicea (~368 C.E.) giving further state tolerance to Christians living along side the pagan worshippers of Mithraism.
The Church historian ‘Eusebius of Caesarea (~325 C.E.) was a rallying contender in aid to the newly formed Christian campaign having his hands in it in several ways. He was a magnificent scholar with a huge literary influence which has created some speculation by various critics that he may have edited bible writings of earlier church fathers to sway support for his emperor’s new found claims in justification for Sunday worship. Here we have Eusebius’s account of Constantine speaking of unanimity regarding Easter against the practice of the Jews;
[Quote]
“At the meeting the question concerning the most holy day of Easter was discussed, and it was resolved by the united judgment of all present, that the feast ought to be kept by all in every place on one and the same day…For we have it in our power, if we abandon their custom, to prolong the due observance of this ordinance to future ages, by a truer order, which we have preserved from the very day of passion until the present time. Let us have nothing in common with the…Jewish crowd; for we have received from our Saviour a different way.” [Quote]
‘Eusebius’: The Life of Constantine Chapter XVIII
From the Nicene and Post-Nicene Fathers of the Christian Church
Philip Schaff & Henry Wace
Eerdmans and T&T Clarke 1997
The Epistle of the Emperor Constantine, Concerning the matters transacted at the Council (Nicea), addressed to those Bishops who were not present; [Quote]
Constantine Agustus to the churches;
“ I now procceed briefly to recapitulate the whole of the preceding. The judgement of all is, that the holy pascal feast should be held on one and the same day ; for, in so holy a matter, it is not right that difference of custom should prevail. It is the more commendable to obey this decree, because it precludes all association with error and with sin. This being the case, receive with gladness the heavenly gift and sacred command; for all that is transacted in the holy councils of the bishops, is sanctioned by the Divine will. Therefore, when you have made known to all our beloved brethren the subject of the epistle, you will be ‘bound to conform to the regular observance of this holy day’, so that when, according to my long cherished desire, I shall be with you, I may be able to celebrate with you this holy festival upon one and the same day…” [End Quote]
Reiterated by Theodoretus Bishop of Cyrus (387 C.E.~ 458 C.E.)
The Greek Ecclesiastical Historians of The First Six Centuries of The Christian Era. Book I Chapter X ‘Esuebius’s Life of Constantine’.