There is no need for work on the Shabbat.
What we do on this day is holy activity, which is of a different essence.
During the week, one is engaged in plowing, in breaking the encrusted soil around the heart, freeing the space for seed.
We are involved with the tasks of Tikkun–repairing the world and sorting out the good, separating out the evil.
All of which is only preparation for the gladness of the Shabbat.
In one moment there is a reversal, an overturning of the profane into the holy.
To be more specific about this reversal, one of the ways it can be recognized is that certain actions, which are unqualified and optional on the weekday, become mitzvot on Shabbat–commandments of the Lord.
The change is a matter of the very formal nature of the Shabbat.
Thus, whereas on ordinary days, one is supposed to eat what is necessary for subsistence, on the Shabbat, eating and the joy of eating are mitzvot.
On the weekday, sleep is a matter of choice or necessity; on Shabbat, it becomes part of the mitzva of rest, and so on.
Which is to say that during the week one relates to action as a means, a vehicle for life to manifest.
One eats in order to live, not as an end in itself; and for many it is simply necessary in order to be able to work or to do whatever one wishes to do.
Whereas on Shabbat, when work and action are revoked, eating becomes a mitzva, a kind of sacrament.
It is part of the process of raising up all the worlds on this day when the profane is transmuted to holiness.
This refers to the truism that the work of the weekday is an extension of the Divine hiddenness.
On Shabbat, all action expresses something of Divine revelation.
As it is written: "And thou shall call the Shabbat a delight" (Isaiah 58:13).
–Rabbi Adin Steinsaltz
From "Hidden Aspects of Shabbat" in The Candle of God by Rabbi Adin Steinsaltz