Does "remember the Sabbath day" imply that the Sabbath day was observed prior to the giving of the Ten Commandments?
The Sabbath day was observed prior to the giving of the Law. When God gave the Israelites manna, He told them to gather the manna for six days, but not to gather on the seventh day. "Then he said to them, "This is what the LORD has said: 'Tomorrow is a Sabbath rest, a holy Sabbath to the LORD. Bake what you will bake today, and boil what you will boil; and lay up for yourselves all that remains, to be kept until morning.'" ... "See! For the LORD has given you the Sabbath; therefore He gives you on the sixth day bread for two days. Let every man remain in his place; let no man go out of his place on the seventh day." So the people rested on the seventh day." (Exodus 16:23, 19-30). Hence, it is perfect legitimate to say that the Lord told the people to keep His law that they had been observing in mind. It is particularly significant for this law because when it was first given, the people had ignored it. "Now it happened that some of the people went out on the seventh day to gather, but they found none. And the LORD said to Moses, "How long do you refuse to keep My commandments and My laws?" (Exodus 16:27-28). Given their track record of "forgetting" the very first week, it is reasonable that the Lord emphasized their need to remember the Sabbath.
In the second telling of the Ten Commandments, the emphasis was not placed on remembering to observe the Sabbath day, but to use the Sabbath day to remember that they were once slaves in Egypt. "Observe the Sabbath day, to keep it holy, as the LORD your God commanded you. Six days you shall labor and do all your work, but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your ox, nor your donkey, nor any of your cattle, nor your stranger who is within your gates, that your male servant and your female servant may rest as well as you. And remember that you were a slave in the land of Egypt, and the LORD your God brought you out from there by a mighty hand and by an outstretched arm; therefore the LORD your God commanded you to keep the Sabbath day" (Deuteronomy 5:12-15). Hence, the Sabbath day was a day of remembrance.
Even if there were no prior instances of observing the Sabbath, it is still proper to give a law and ask the people to remember it in the future. Jesus essentially did this when establishing his memorial feast. "And He took bread, gave thanks and broke it, and gave it to them, saying, "This is My body which is given for you; do this in remembrance of Me."" (Luke 22:19). Though Christ's body had yet to be sacrificed at this point in time, Jesus told his disciples to partake of it remembering what he did for them. In the future, the disciples did partake of the Lord's Supper and at that time they were able to look back at what Christ had done, but when the command was originally given it was still a future event. "For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me." In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me." For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes" (I Corinthians 11:23-26).
The Israelites were to put tassels on their garments to help them remember the laws of God. "And you shall have the tassel, that you may look upon it and remember all the commandments of the LORD and do them, and that you may not follow the harlotry to which your own heart and your own eyes are inclined, and that you may remember and do all My commandments, and be holy for your God" (Numbers 15:39-40). Yet, at the time this law was given, all the laws had not been presented to the Israelites. Still, in the future they would be able to look at their tassels and think back to the laws that had been given even if some were given after the command to hang tassels on their garments.
In advising young people, Solomon told them, "Truly the light is sweet, and it is pleasant for the eyes to behold the sun; but if a man lives many years and rejoices in them all, yet let him remember the days of darkness, for they will be many. All that is coming is vanity. Rejoice, O young man, in your youth, and let your heart cheer you in the days of your youth; walk in the ways of your heart, and in the sight of your eyes; but know that for all these God will bring you into judgment" (Ecclesiastes 11:7-9). Though these young people have yet to see the sorrows of the world, Solomon tells them to enjoy the pleasant times but always to remember the sorrowful times that will come to them.
We do much the same with our own children. How many mothers have told their children. "Just remember this when you have children, then you'll understand"? The act of remembering is to look back, but the command to remember can deal with a future time when remembrance will take place.
Therefore, just because the Lord told Israel to remember the Sabbath day, it does not imply that the Sabbath day was an observance that had been going on for a long time.