That is probably the most important piece of evidence - the recognition of Sunday as the day of worship over Saturday (sabbath) early in the life of the Church. We can quibble in the 21st century over days, hours, Jewish idioms, etc - but the fact that Sunday became the day of worship is undeniable - centuries before it was cautified by later church councils.
Ignatius was one of the earliest witnesses after the apostles of worship on the first day of the week. He contrasted the seventh day, the Jewish Sabbath, with the first day of the week, the day Christ resurrected:
“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 after a spiritual manner, rejoicing in meditation on the law, not in relaxation of the body, admiring the workmanship of God, and not eating things prepared the day before, nor using lukewarm drinks, and walking within a prescribed space…let every friend of Christ keep THE LORD’S DAY as a festival, THE RESURRECTION-DAY, the queen and chief of all the days.”
He wrote: “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 OBSERVANCE OF THE LORD’S DAY...”
Ignatius was referring to the old covenant when he said, “brought up in the ancient order of things” He was arguing that since the early Jewish church kept “the Lord’s day,” meaning the first day of the week when Christ was resurrected, we should do likewise.
These quotes indicate personal knowledge by Ignatius that the early church came to see the day of Jesus’ resurrection as a special day for Christians. It was the day set apart for resurrection observance. No doubt Ignatius’ term “the Lord’s Day,” originated with John in Rev. 1:10. By the time John wrote Revelation, very late in the first century, it must have become commonplace to refer to the first day of the week as “the Lord’s Day.”