The change came after the time of the church fathers and it was officiated by Constantine.
However, I we are forgiven by Christ for our errors in understanding and that as long as you are a disciple of Him, you are saved by Him.
Wasn’t Constantine a Roman?
Entirely false. I just provided a link with early Christian writings starting in the last 1st century all referring to worship on the "Lord's Day," aka the first day of the week.
"[L]et every friend of Christ keep the Lord's Day as a festival, the resurrection-day, the queen and chief of all the days of the week. (Ignatius, Epistle to the Magnesians, chp 9. Ante-Nicene Fathers , vol. 1, pg. 62-63.)