Remember the Sabbath-day and keep it holy
Prior to the industrial revolution, farmers and tradesmen worked 6 days a week, leaving the seventh day dedicated to physical rest and spiritual renewal. In our modern society, many businesses are open 24 hours/7days a week/365 days per year, requiring employees to work on Sunday, the traditional Sabbath day. For the modern Christian, then, keeping this Commandment is impossible if looked at in the strictest, most literal sense; however, when looked at in light of Luther’s explanation, we find that the Commandment does not require that a specific day be set aside. Rather, the Christian must set aside time for physical rest and, more importantly, devote time to God, hearing and retaining his Word.
Q. What does this mean?
A. We are to fear and love God, so that we will not look down on preaching or God's Word, but consider it holy, listen to it willingly, and learn it.
From Luther’s Large Catechism
79] The word holy day (Feiertag) is rendered from the Hebrew word sabbath which properly signifies to rest, that is, to abstain from labor. Hence we are accustomed to say, Feierabend machen [that is, to cease working], or heiligen Abend geben [sanctify the Sabbath]. 80] Now, in the Old Testament, God separated the seventh day, and appointed it for rest, and commanded that it should be regarded as holy above all others.
83] But to grasp a Christian meaning for the simple as to what God requires in this commandment, note that we keep holy days not for the sake of intelligent and learned Christians (for they have no need of it [holy days]), but first of all for bodily causes and necessities, which nature teaches and requires; for the common people, man-servants and maid-servants, who have been attending to their work and trade the whole week, that for a day they may retire in order to rest and be refreshed.
84] Secondly, and most especially, that on such day of rest (since we can get no other opportunity) freedom and time be taken to attend divine service, so that we come together to hear and treat of God's Word, and then to praise God, to sing and pray.
85] However, this, I say, is not so restricted to any time, as with the Jews, that it must be just on this or that day; for in itself no one day is better than another; but this should indeed be done daily; however, since the masses cannot give such attendance, there must be at least one day in the week set apart. But since from of old Sunday [the Lord's Day] has been appointed for this purpose, we also should continue the same, in order that everything be done in harmonious order, and no one create disorder by unnecessary innovation.
87] Accordingly, when asked, What is meant by the commandment: Thou shalt sanctify the holy day? answer: To sanctify the holy day is the same as to keep it holy. But what is meant by keeping it holy? Nothing else than to be occupied in holy words, works, and life. For the day needs no sanctification for itself; for in itself it has been created holy [from the beginning of the creation it was sanctified by its Creator]. But God desires it to be holy to you. Therefore it becomes holy or unholy on your account, according as you are occupied on the same with things that are holy or unholy.
Large Catechism Text in the public domain. http://bookofconcord.org/lc-3-tencommandments.php