The perfection of the universe is fittingly ascribed to the Seventh Day; for the perfection of things is twofold: first, there is the perfection of the substance, which arises from the integrity of the parts; and secondly, that of the end or operation, i.e. whatever is arrived at by means of work.
The first of these perfections is the cause of the second; for form is the principle of the operation and belongs to the first institution of things, which is ascribed to the Seventh Day; while the second is reserved for the end of the world. Hence the perfection of Nature was completed on the Seventh Day; that of Grace in the Incarnation of Christ; and that of Glory, which consists in the perfect Beatitude of the saints, will be completed at the end of the world.
And God rested from creating anything new, since everything pre-existed either materially, or causally, or according to similitude. Nor did God rest only by ceasing from new creations, but in Himself; for He is blessed in Himself, having no need of any other; hence Scripture says: " He rested from them." And since God ceased from creating new things on the Seventh Day, He blessed that which He preserves by administration, and sanctified that Day; because in it all things rest; by which is to be understood the sanctification of all creatures. And He gave them the power of multiplying.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni