There are two aspects to the Shabbat, one negative and one positive. On one hand, we refrain from specific types of labor, such as writing, kindling a fire, or cooking and baking.  On the positive side, we endeavor to make the seventh day one of spiritual creativity and activity.

From the very first, G-d imbued the Shabbat with qualities not inherent in the other days of the week.  It is a special entity, a unique segment of time fashioned with properties that allow it to accomplish its special task.  Sunday through Friday were given us to earn our livelihood; the Shabbat was created in order to ensure that the Jew have an opportunity to refresh his spirit every seventh day.

In his classic work, The Kuzari, Rabbi Judah Halevi quotes the king of the Kuzars’ observation about the Shabbat:

I have been thinking for some time about your fate, and have come to the conclusion that G-d has some secret in preserving you, namely the observance of the Sabbath and the Three Festivals…

Without your observance of the Sabbath… as commemoration of the Creation, of the Exodus from Egypt, and the Giving of the Law on Mount Sinai, the nations that ruled over you would have enslaved you, and used you as mercenary soldiers…  Without the Sabbath not a single one of you would have worn a clean garment and you would not have enjoyed a single day in your life.  You would have had no time to congregate to study your Torah, because of your debased state, stemming from the continuing exile.

…the Sabbaths and holidays amount to approximately one-sixth of the year.  During these days you experience physical and spiritual rest of a degree that even kings lack… when a king needs to do something on his day of rest, he does not refrain… from exerting himself, so that he does not have spiritual rest…

The Kuzari, Essay 3, Paragraph 10

Rabbi Samson Rephael Hirsch writes:

Were man left entirely to his own devices, without a Divine command to observe the Sabbath, he would not have the discipline and will-power to keep the Seventh Day as ordained by the Torah.  As a result, man would be left with no day of spiritual rest…

How great is the blessing which this day brings to those who observe it!  Were it not for the strictly imposed cessation from industrious, creative work imposed upon man from Above, when would he ever give himself a much-needed respite from his labors?  When would he find the time to devote his full attention to his family, to his own spiritual being, to his relationship with his Maker?  When would he find the time to advance his knowledge of G-d’s Torah?

When you have a free moment, you say?  But just when do you have a free moment?

Without the Shabbat, without respite, man labors endlessly, and is subject to on-going worries which gnaw away at his mind.  He will not grow wealthy or become contented by being overburdened.

The Shabbat is an agent sent to him from Heaven.  The Sabbath descends upon the world and calls out in G-d’s name: “Cease!  Rest from your harried labors!  Do not extend your hand endlessly in an effort to earn your livelihood.  If you devote your mental powers only to seeking your daily bread, you are eternally sinning to your soul and to the souls of your household.”

The repose of the Shabbat includes a rest and renewal of the spirit.  We should so remove ourselves from the pressures of workday concerns that we feel as though our desks are completely cleared.  There is nothing waiting for our attention.  All our tasks have been completed, and we are free to devote ourselves entirely to nourishing our souls rather than our bodies.  As the Minchah prayer for Shabbat describes it:

…a calmness of truth and faith, a calmness of peace and serenity and tranquility and security, a perfect rest, which You desire.

Our prayers describe the qualities of the restful calm which should prevail on Shabbat.  It is a repose which finds favor in the eyes of our Creator, a tranquility in which He delights and which He bestows on His children.

The soul cannot be completely restful unless it is occupied with spiritual concerns.  The Shabbat comes each week to suffuse our soul with its blessings of true peace and harmony.