Below one will see that nearly 3 centuries after the crucifixion, Christians were giving reverential treatment to 2 days- Sabbath (Saturday) & The "Lord's day" (by which they likely meant Sunday).....so Sunday sacredness had been creeping in yet they still
held to the Sabbath as well....
"[c. 250-300 AD Apostolic Constitutions:] We enjoin you to fast every fourth day of the week, and every day of the preparation, and the surplusage of your fast bestow upon the needy; every Sabbath-day excepting one, and every Lord's day, hold your solemn assemblies, and rejoice: .....—bk. 5, sec. 3, xx." ADL
"[c. 250-300 AD Apostolic Constitutions:] We enjoin you to fast every fourth day of the week, and every day of the preparation, and the surplusage of your fast bestow upon the needy; every Sabbath-day excepting one, and every Lord's day, hold your solemn assemblies, and rejoice: .....—bk. 5, sec. 3, xx." ADL
But the seventh day is the sabbath of the LORD thy God:
Exodus 20:10