In my experience as a Born-Again Protestant (with a background from Pentecostal and Baptist congregations) and study, at first it seemed that Sunday was the Sabbath. Culturally where I'm from Sunday is seen as the family day, so it's a one-all for church, family fellowship, and rest. It's really impossible to rest when you're busy prepping to go somewhere then hang out with family afterwards.

However, Colossians 2 seems to put a damper on this:

“And you, being dead in your trespasses and the uncircumcision of your flesh, He has made alive together with Him, having forgiven you all trespasses, having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross. Having disarmed principalities and powers, He made a public spectacle of them, triumphing over them in it. So let no one judge you in food or in drink, or regarding a festival or a new moon or Sabbaths, which are a shadow of things to come, but the substance is of Christ. Let no one cheat you of your reward, taking delight in false humility and worship of angels, intruding into those things which he has not seen, vainly puffed up by his fleshly mind, and not holding fast to the Head, from whom all the body, nourished and knit together by joints and ligaments, grows with the increase that is from God.” – Colossians 3:13-19 [NKJV]

You see, the Colossian church mixed various religions (Essenism, Merkabah mysticism, Gnosticism, Judaism, and Greek paganism) along with Christianity. This included the false belief that you must strictly observe Jewish laws, festivals, and live a strictly dietary life - among others - to attain salvation.

And what did Paul have to say about them?

“Therefore, if you died with Christ from the basic principles of the world, why, as though living in the world, do you subject yourselves to regulations — ‘Do not touch, do not taste, do not handle,’ which all concern things which perish with the using — according to the commandments and doctrines of men? These things indeed have an appearance of wisdom in self-imposed religion, false humility, and neglect of the body, but are of no value against the indulgence of the flesh.” – Colossians 3:20-24 [NKJV]

Basically, these practices were not only wrong but completely useless in attaining salvation, which also applies to taking your rest day. Salvation is by faith alone, which = good works, not faith + good works. You take observe the Sabbath because you love God and want to obey Him first AND because you need a break.

Therefore, the Sabbath isn't a set day of the week for us since we're not Jews. A correct way of interpreting it is to just take one day off a week - be it a Monday, Tuesday, etc. - in order to honor that day of rest as different from the other 6 days that were really busy.

Using the Westminster Confession of Faith to present a Reformed position, Chapter XXI, paragraph VIII states:

This Sabbath is to be kept holy unto the Lord when men, after a due preparing of their hearts, and ordering of their common affairs beforehand, do not only observe an holy rest all the day from their own works, words, and thoughts about their worldly employments and recreations; but also are taken up the whole time in the public and private exercises of his worship, and in the duties of necessity and mercy.

This presents the Sabbath's purpose of getting people out of their self-satisfaction and self-importance ("own works, words, and thoughts") and directed toward God ("worship") while being godly in not neglecting needs of self or others ("necessity and mercy"--"duties of necessity" can be somewhat vague but I believe would include certain work of doctors, police, etc.).

(By the way, in paragraph VII, the writers of the WCF present the belief that Sunday is the post-resurrection/Christian Sabbath day.)

Even in cultures with 5-day work weeks (which at least some Reformed persons believe would also violate the 4th commandment, if two rest days are used), there are "slave-driver" employers and workaholics for which the imposition of a ceasing would be a mercy even from an atheistic perspective.

(By the way, the Exodus version draws on the creation relation--all mankind, all time (at least for some interpreters)--and presents it as a blessing; the Deuteronomy version relates it to salvation from slavery and as a covenant command as well as emphasizing likeness to the slave--reminiscent, to me, of Col. 4:1 "Masters, provide your slaves with what is right and fair, because you know that you also have a Master in heaven"(NIV).)

The Sabbath has several purposes, including: to reinforce trusting that God will provide (cf. Manna gathering in Ex. 16:19-30), to provide/enforce freedom from work (i.e., work is not to be the master), to provide/enforce rest, to have a dedicated regular (habitual) time to remember who God is and what God has done, to develop a future-oriented mindset (Sabbath requires preparation and is a guaranteed future time, perhaps hinting at anticipation of (and preparation for) the time of the new heaven and new earth with a sense of certainty, the fullness of rest is as certain as one day following another).

Even for Christians who reject Sabbath observance as a moral law (I think St. Augustine was one such, so this certainly appears to be a "disputable matter"), the principles of not abusing employees, of regularly remembering that God is God (Creator and Master), of depending on God, and of resting--these principles would still apply. In addition, such Christians might adopt the pattern of Sabbath (with its related purposes such as those mentioned above) as a practical aid toward sanctification much like a dedicated time of Bible study.

Given that entire books have been written on this subject, a StackExchange answer is not likely to be especially comprehensive.