Adam Clarke on Exodus 16:23

To-morrow is the rest of the holy Sabbath - There is nothing either in the text or context that seems to intimate that the Sabbath was now first given to the Israelites, as some have supposed: on the contrary, it is here spoken of as being perfectly well known, from its having been generally observed. The commandment, it is true, may be considered as being now renewed; because they might have supposed that in their unsettled state in the wilderness they might have been exempted from the observance of it. Thus we find, 1. That when God finished his creation, he instituted the Sabbath; 2. When he brought the people out of Egypt, he insisted on the strict observance of it; 3. When he gave the Law, he made it a tenth part of the whole, such importance has this institution in the eyes of the Supreme Being! On the supposed change of the Sabbath from what we call Sunday to Saturday, effected on this occasion, See Clarke's note on Deu 5:15.