Even in the NT there are signs Sunday was becoming the Christian holy day. Corinthians made collection for Jerusalem church. C. Paul and the Sabbath. He condemns observance of days, months, years, etc, as legalism. Stay away from such disputes.
Every day is holy.
Transformation of Sabbath into Sunday, into Sabbath. By the second century, the Lord's Day had won over the Sabbath. Sabbath keeping was no longer binding. Most Christians would not be allowed to. Even when they were, they didn't.