Ramban & Ritva
Safeguard the day of Shabbos to make it holy as Hashem your G-d commanded you. (Devarim 5:12)
The Ramban writes (Emor, Vayikra 23:24) that the shevusin – the specific forms of rest, and acts that we are to refrain from – are Torah-mandated laws. Without them, the entire day of Shabbos would be spent engaged in otherwise permitted forms of labor such as commercial transactions and arranging merchandise. However, the Torah left it to the Sages to enumerate and explain the details and specifics of these forbidden activities. (See further in Ramban’s commentary to the Torah.) The Ritva (Maseches Rosh Hashanah 32b) greatly praises this teaching.
The Ritva writes that the term shevus d’rabbanan – of the rabbis – does not imply that the concept of shevus (mandated refraining from non-malacha activities) does not exist at the Torah - commandment level. For if that was true, Shabbos would have the appearance of an ordinary weekday. Stores would be open, as would warehouses of grain and wine. People would carry objects back and forth from house to house through the carmelis domain; they would weigh, measure, and inventory things. How could it be that the Torah forbade us to transfer an object as small as a dried fig or date from a private domain to a pubic one while permitting all this great labor that would cause the Shabbos to cease being a day of rest?!
Rather, the explanation is that the Torah’s general prohibition of creative labor includes a positive commandment of shevus – rest, i.e. rest from labor. This is a general directive to ensure that Shabbos will not be transformed into an ordinary profane weekday. However, the delineation of the specific details needed to prevent Shabbos from becoming like an ordinary profane day is assigned to the rabbis and sages. ... This is a gem that we have received from the Ramban.]
The purpose of Shabbos is to publicize the creation and renewed creation of the world by Hashem ex nihilo – out of nothing. It is testimony to the creation of the world. Another, individual-oriented reason for Shabbos observance is to allow us to set aside one weekly day of rest from labor and mundane work and turn our hearts towards the knowledge of Hashem and His ways and to study the Torah, so as not to become overly immersed in the ways of the physical world.
We find that the Talmud Yerushalmi (Maseches Shabbos 15:3) teaches that the days of Shabbos were given only for the purpose of our studying Torah on them. This source conforms with the second reason we cited. A second statement in the Yerushalmi, that Shabbos was given to us for eating and drinking, corresponds to the first reason of reminding us of Creation ex nihilo. It teaches us that compared to Creation itself, anything else is easy for Hashem; therefore we needn’t worry about our sustenance and livelihood. Eat and drink and Hashem will pay for it all, so don’t worry about tomorrow... If all the categories of mandated rest known as shevusin would be permitted, the entire day would be spent performing these permitted forms of labor and mundane work with great toil, and what time would there be for the study of Torah?!
The Torah thus alluded to this by stating “Shamor es yom HaShabbos l’kadsho – safeguard the day of Shabbos to make it holy.” Shamor refers to the fences and enactments enumerated by the Sages to safeguard and protect the mitzvos (Meshech Chochma, Eikev).
