The Torah repeatedly uses the verb “keep” (shemor) when instructing about the Sabbath. While this verb is also used for the entirety of Torah and mitzvahs, its use in the context of a particular commandment — here, repeated three times in the context of the Sabbath — is unique. This language has taken root in our colloquial speech, as we refer to one who observes the Sabbath as a “shomer Shabbat.”
The use of this term teaches us to regard the Sabbath as a precious possession given to us and entrusted to our care. Just as a watchman must guard the object of its care, take care not to be negligent, and never tamper with it, the Jewish People is instructed to be scrupulous and vigilant in keeping away and fending off anything that might damage this treasure.
G-d instructs us, “Only keep my Sabbaths!” The plural indicates that what is entrusted to our safekeeping is not the Sabbath in general, but that each and every Sabbath day is so entrusted as a unique asset. The word “only,” explain our Sages, teaches that the Sabbath is given priority over the work of the Tabernacle. The “safeguarding,” then, is not against violation by personal, profane activity or ordinary occupational pursuits, but rather against the sublime work of constructing a dwelling for G-d! Thus, the priority of Sabbath observance over all other endeavors less sacred is impressed with greater impact.
“This” — the safekeeping itself — is a “sign” between G-d and Israel so that we may know G-d sanctifies us. (Shemot 31:13). By our safekeeping — by refraining from constructive work on Sabbath —we lay ourselves, our whole world, and all the powers we have been granted, to shape the world in homage before G-d.
Ever since the world’s existence, the Sabbath was known as a memorial to G-d’s creation of the world. But it existed only as an abstract idea. Because it lacked a visible expression — a symbol — it disappeared from man’s consciousness.
When Israel was chosen as an instrument for G-d’s rule — to acknowledge and spread the knowledge of Him as Master and Ruler of mankind — He gave Sabbath the concrete symbol of prohibiting creative activity. This was a sign “between G-d and Israel” of the mutual relationship, so that we may know He has chosen Israel to be that instrument.
Sources: Commentary, Shemot 31:13