John Gill Commentary Deuteronomy 5:12

"Observe the sabbath day, to keep it holy, as Jehovah thy God commanded thee." — Deuteronomy 5:12 (ASV)

Ver. 12,13. Keep the sabbath day to sanctify it
Or observe it, by setting it apart as a time of natural rest, and for the performance of holy and religious exercises; see (Exodus 20:8) , where the phrase is a little varied, "remember the sabbath day to keep it holy"; it having been instituted before:

as the Lord your God has commanded you ;
not at Sinai only, for the same might then have been observed of all the rest of the commands, but before the giving of the law, at the first of the manna; see (Exodus 16:23) .