Whatever day one chooses to worship God is not a great issue for me.
It ought to be, since a Christian is to worship God 24/7/365, which includes obedience to His commandments in His salvation offered in Christ Jesus.
If a person (knowingly, that is presumptuously) does not rest as God commands on the 7th day, as He did, then they are worshipping something or someone else other than the LORD of the Sabbath.
I am not speaking of the ignorant (not knowing). God winks at ignorance, but also stated by Paul:
Act 14:15 And saying, Sirs, why do ye these things? We also are men of like passions with you, and preach unto you that ye should turn from these vanities unto the living God, which made heaven, and earth, and the sea, and all things that are therein:
Act 14:16 Who in times past suffered all nations to walk in their own ways.
Exo 20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
Act 17:30 And the times of this ignorance God winked at; but now commandeth all men every where to repent:
Then see Acts 15:13-19, Amos 9:11-12, and then Isaiah 56:1-8 and John 10:16.