1. The sabbath was a part of the Mosaic law until Christ came to fulfill
the law. The 'rest' of the Old Testament is now in Jesus, who is our
rest! Isa 28:10 For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little: Isa 28:11 For with stammering lips and another tongue will he speak to this people. Isa 28:12 To whom he said, This is the rest wherewith ye may cause the weary to rest; and this is the refreshing: yet they would not hear.
Mat 11:28 Come unto me, all ye that labour and are heavy laden, and I will give you rest. Mat 11:29 Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls. Mat 11:30 For my yoke is easy, and my burden is light.
2. We are admonished by Paul about holy days - does not sound like keeping a particular day holier than another is a big deal!
Rom 14:5 One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind.
Col 2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Col 2:17 Which are a shadow of things to come; but the body is of Christ.
3. I would certainly think that if keeping the sabbath was important, it
would have been mentioned in Acts 15 when the apostles convened in
order to discuss what requirements should be placed on the Gentiles :
Acts 15:28 For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things; Act 15:29 That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
4. As for Saturday vs Sunday, it's a moot subject since we don't observe
it as it was observed in the Old Testament. But it's obvious in
scripture that the disciples worshiped on the first day of the week and
not the 7th day of the week but their reason for doing this was not to 'keep' the sabbath.
Act_20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
1Co_16:2 Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.