I would recommend reading Sabbath in Christ by Dale Ratzlaff.
I was a former Sabbathkeeper, as a member of Worldwide Church of God (Armstrongism), for over a decade.
My conviction is that the Sabbath is not applicable per Colossians 2:16-17. In addition, Sabbath-breaking was not mentioned on any of the sin lists of the Gentiles. In addition, the Sabbath was a sign of the Old Covenant between God and Israel and not God and mankind, therefore it never was applicable to those who were not under the Old Covenant.
SDAs spend a lot of effort claiming that the weekly Sabbath isn't mentioned in Colossians 2:16-17 but they cannot prove it. The word Sabbath is sabbaton, which has a weekly context. Indeed, it is translated "week" eight times in the NT. For example, mia ton sabbaton means "first day of the week" where mia means one and sabbaton means week.
One basic question that should be asked is whether the Old Covenant is in effect, and whether the Sabbath, festivals, and clean/unclean meat laws are moral in nature, or were part of the "boundary markers" which separated ancient Israel from the Gentile nations around it.
This is a very long discussion, though, and usually those in the discussion have already made their minds up. Unless they are claiming that the Sabbath is a requirement for salvation, and everyone else is unsaved, I typically won't waste the time explaining the problems with their position. Most Sabbath keepers in the chat room don't make such claims, but there are cultic individuals which make such claims (Armstrongites for example).
For every point the Sabbathkeepers make, a counter-point can be made.
I would recommend this video to understand the Abrahamic, Old, and New Covenant, and why the Sabbath is included in Colossians 2:16-17. Also, is the Ten Commandments a separate group of laws which are still applicable, while the Book of the Covenant is not applicable?
It is based somewhat on the "Sabbath in Christ" book by Dale Ratzlaff. Dale is a former SDA pastor and teacher. I highly recommend the book as well. I don't agree with one minor point Dale makes, but I agree with the overall thesis.
https://www.youtube.com/playlist?list=PLKnzCuBYVJm5aePMh-rTUtoi2-QNiKDs2
At any rate, individuals should follow their own conscience on the issue of days, and I respect the willingness of Sabbathkeepers to obey God in what they understand to be true, whether I agree or not. Hopefully all Christians have this spirit. Read Romans 14 regarding these issues of differences of opinions on days and food.