The literal translation of the phrase, “in the course of time,” is, “at the end of days.” The most likely and reasonable meaning of this phrase is, “at the end of the seven-day week,” which strongly suggests that the pattern of the Sabbath was known about and practiced long before Sinai.
4 Then the Lord said to Moses, “Behold, I am about to rain bread from heaven for you, and the people shall go out and gather a day’s portion every day, that I may test them, whether they will walk in my law or not. 5 On the sixth day, when they prepare what they bring in, it will be twice as much as they gather daily.”
23 he said to them, “This is what the Lord has commanded: ‘Tomorrow is a day of solemn rest, a holy Sabbath to the Lord; bake what you will bake and boil what you will boil, and all that is left over lay aside to be kept till the morning.’” 24 So they laid it aside till the morning, as Moses commanded them, and it did not stink, and there were no worms in it. 25 Moses said, “Eat it today, for today is a Sabbath to the Lord; today you will not find it in the field. 26 Six days you shall gather it, but on the seventh day, which is a Sabbath, there will be none.”
27 On the seventh day some of the people went out to gather, but they found none. 28 And the Lord said to Moses, “How long will you refuse to keep my commandments and my laws? 29 See! The Lord has given you the Sabbath; therefore on the sixth day he gives you bread for two days. Remain each of you in his place; let no one go out of his place on the seventh day.” 30 So the people rested on the seventh day.
The most reasonable interpretation of this passage is that the Sabbath was an institution that the Israelites would have already been familiar with, which demonstrates that the Sabbath was known about and practiced long before Sinai.