The historic Reformed position is that the sabbath is a creation ordinance. That the Sabbath is a creation ordinance means that it was a command that Adam, even in his pre-fallen state, would have known about and obeyed, and that it is a universal, eternal command that every human everywhere is bound to obey at all times.
Creation ordinances are mandates, or commands or principles that God gave to us in our original, our prefallen, our unfallen state. They are designed to promote God’s glory and to practically express what it means for us to be made in God’s image.J. Ligon Duncan
There are strong arguments that the sabbath is a creation ordinance.
1 Thus the heavens and the earth were finished, and all the host of them. 2 And on the seventh day God finished his work that he had done, and he rested on the seventh day from all his work that he had done. 3 So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation.Genesis 2:1-3
In Genesis 2:1-3, the pattern of creation is that God spent six days creating the universe, and then God rested on the seventh day. The passage says that “God blessed the seventh day and made it holy.”
The most reasonable interpretation of this phrase is that God’s pattern of six days of work and one day of rest on this seventh day, which God blessed and made holy, is a pattern that humans should follow. If God followed this pattern and blessed and made holy the seventh day, then this naturally leads to the interpretation that humans should also follow this pattern.
22 On the sixth day they gathered twice as much bread, two omers each. And when all the leaders of the congregation came and told Moses, 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.Exodus 16:22-30
Exodus 16:22-30 is significant because it demonstrates that the Sabbath was a command that was given before Sinai.
To read more about the Reformed position concerning the Sabbath, check out the following resources: