Objection 1. It would seem that God did not rest on the seventh day from all His work. For it is said (John 5:17), "My Father worketh until now, and I work." God, then, did not rest on the seventh day from all His work.
Objection 2. Further, rest is opposed to movement, or to labor, which movement causes. But, as God produced His work without movement and without labor, He cannot be said to have rested on the seventh day from His work.
Objection 3. Further, should it be said that God rested on the seventh day by causing man to rest; against this it may be argued that rest is set down in contradistinction to His work; now the words "God created" or "made" this thing or the other cannot be explained to mean that He made man create or make these things. Therefore the resting of God cannot be explained as His making man to rest.
On the contrary, It is said (Genesis 2:2): "God rested on the seventh day from all the work which He had done."
I answer that, Rest is, properly speaking, opposed to movement, and consequently to the labor that arises from movement. But although movement, strictly speaking, is a quality of bodies, yet the word is applied also to spiritual things, and in a twofold sense. On the one hand, every operation may be called a movement, and thus the Divine goodness is said to move and go forth to its object, in communicating itself to that object, as Dionysius says (Div. Nom. ii). On the other hand, the desire that tends to an object outside itself, is said to move towards it. Hence rest is taken in two senses, in one sense meaning a cessation from work, in the other, the satisfying of desire. Now, in either sense God is said to have rested on the seventh day.
First, because He ceased from creating new creatures on that day, for, as said above (1, ad 3), He made nothing afterwards that had not existed previously, in some degree, in the first works; secondly, because He Himself had no need of the things that He had made, but was happy in the fruition of Himself. Hence, when all things were made He is not said to have rested "in" His works, as though needing them for His own happiness, but to have rested "from" them, as in fact resting in Himself, as He suffices for Himself and fulfils His own desire. And even though from all eternity He rested in Himself, yet the rest in Himself, which He took after He had finished His works, is that rest which belongs to the seventh day. And this, says Augustine, is the meaning of God's resting from His works on that day (Gen. ad lit. iv).
Reply to Objection 1. God indeed "worketh until now" by preserving and providing for the creatures He has made, but not by the making of new ones.
Reply to Objection 2. Rest is here not opposed to labor or to movement, but to the production of new creatures, and to the desire tending to an external object.
Reply to Objection 3. Even as God rests in Himself alone and is happy in the enjoyment of Himself, so our own sole happiness lies in the enjoyment of God. Thus, also, He makes us find rest in Himself, both from His works and our own. It is not, then, unreasonable to say that God rested in giving rest to us. Still, this explanation must not be set down as the only one, and the other is the first and principal explanation.
Source: Summa Theologica (New Advent)