Objection 1. It would seem that the completion of the Divine works ought not to be ascribed to the seventh day. For all things that are done in this world belong to the Divine works. But the consummation of the world will be at the end of the world (Matthew 13:39-40). Moreover, the time of Christ's Incarnation is a time of completion, wherefore it is called "the time of fulness [Vulgate: 'the fulness of time']" (Galatians 4:4). And Christ Himself, at the moment of His death, cried out, "It is consummated" (John 19:30). Hence the completion of the Divine works does not belong to the seventh day.
Objection 2. Further, the completion of a work is an act in itself. But we do not read that God acted at all on the seventh day, but rather that He rested from all His work. Therefore the completion of the works does not belong to the seventh day.
Objection 3. Further, nothing is said to be complete to which many things are added, unless they are merely superfluous, for a thing is called perfect to which nothing is wanting that it ought to possess. But many things were made after the seventh day, as the production of many individual beings, and even of certain new species that are frequently appearing, especially in the case of animals generated from putrefaction. Also, God creates daily new souls. Again, the work of the Incarnation was a new work, of which it is said (Jeremiah 31:22): "The Lord hath created a new thing upon the earth." Miracles also are new works, of which it is said (Ecclesiasticus 36:6): "Renew thy signs, and work new miracles." Moreover, all things will be made new when the Saints are glorified, according to Apocalypse 21:5: "And He that sat on the throne said: Behold I make all things new." Therefore the completion of the Divine works ought not to be attributed to the seventh day.
On the contrary, It is said (Genesis 2:2): "On the seventh day God ended His work which He had made."
I answer that, The perfection of a thing is twofold, the first perfection and the second perfection. The 'first' perfection is that according to which a thing is substantially perfect, and this perfection is the form of the whole; which form results from the whole having its parts complete. But the 'second' perfection is the end, which is either an operation, as the end of the harpist is to play the harp; or something that is attained by an operation, as the end of the builder is the house that he makes by building. But the first perfection is the cause of the second, because the form is the principle of operation. Now the final perfection, which is the end of the whole universe, is the perfect beatitude of the Saints at the consummation of the world; and the first perfection is the completeness of the universe at its first founding, and this is what is ascribed to the seventh day.
Reply to Objection 1. The first perfection is the cause of the second, as above said. Now for the attaining of beatitude two things are required, nature and grace. Therefore, as said above, the perfection of beatitude will be at the end of the world. But this consummation existed previously in its causes, as to nature, at the first founding of the world, as to grace, in the Incarnation of Christ. For, "Grace and truth came by Jesus Christ" (John 1:17). So, then, on the seventh day was the consummation of nature, in Christ's Incarnation the consummation of grace, and at the end of the world will be the consummation of glory.
Reply to Objection 2. God did act on the seventh day, not by creating new creatures, but by directing and moving His creatures to the work proper to them, and thus He made some beginning of the "second" perfection. So that, according to our version of the Scripture, the completion of the works is attributed to the seventh day, though according to another it is assigned to the sixth. Either version, however, may stand, since the completion of the universe as to the completeness of its parts belongs to the sixth day, but its completion as regards their operation, to the seventh. It may also be added that in continuous movement, so long as any movement further is possible, movement cannot be called completed till it comes to rest, for rest denotes consummation of movement. Now God might have made many other creatures besides those which He made in the six days, and hence, by the fact that He ceased making them on the seventh day, He is said on that day to have consummaed His work.
Reply to Objection 3. Nothing entirely new was afterwards made by God, but all things subsequently made had in a sense been made before in the work of the six days. Some things, indeed, had a previous experience materially, as the rib from the side of Adam out of which God formed Eve; whilst others existed not only in matter but also in their causes, as those individual creatures that are now generated existed in the first of their kind. Species, also, that are new, if any such appear, existed beforehand in various active powers; so that animals, and perhaps even new species of animals, are produced by putrefaction by the power which the stars and elements received at the beginning. Again, animals of new kinds arise occasionally from the connection of individuals belonging to different species, as the mule is the offspring of an ass and a mare; but even these existed previously in their causes, in the works of the six days. Some also existed beforehand by way of similitude, as the souls now created. And the work of the Incarnation itself was thus foreshadowed, for as we read (Philippians 2:7), The Son of God "was made in the likeness of men." And again, the glory that is spiritual was anticipated in the angels by way of similitude; and that of the body in the heaven, especially the empyrean. Hence it is written (Ecclesiastes 1:10), "Nothing under the sun is new, for it hath already gone before, in the ages that were before us."
Source: Summa Theologica (New Advent)