Ignatius was the bishop of Antioch during the time of Trajan (A.D. 98-117). His anti-Judaic writings demonstrate, first, that at that time there were many early Christians keeping Jewish customs, and secondly, the clear schism that was developing between Christianity as a sect of Judaism and Christianity as a new, man-made religion.
From C.S. Mosna, Storia della Domenica, p.95:
The bishop argues “against the Judaizing tendencies of his territory, which, not far geographically from Palestine, had suffered the influences of the synagogue and of the Judaeo-Christians.”
From Pseudo-Ignatius, Epistle to the Magnesians 9, ANF I, pp. 62-63:
Let us therefore no longer keep the Sabbath after the Jewish manner, and rejoice in days of idleness… but let every one of you keep the Sabbath in a spiritual manner, rejoicing in the meditation on the law, not in the relaxation of the body, admiring the workmanship of God, and not eating things prepared the day before, nor using lukewarm drinks, nor walking within a prescribed space, nor finding delight in dancing and plaudits which have no sense in them.”
Here we can see the first beginnings of a move of the early Christian church away from Judaism, away from a seventh day Sabbath. As we move on to other anti-Judaic early church fathers, Barnabas (the epistle of Barnabas is dated by scholars between A.D. 130-138) and Justin Martyr (Dialogues with Trypho are dated between A.D. 138-161), we can see an even more developed position of anti-Judaism as well as a substitution of Sunday as the preferred day of worship.