Objection 1. It would seem that these days are not sufficiently enumerated. For the work of creation is no less distinct from the works of distinction and adornment than these two works are from one another. But separate days are assigned to distinction and to adornment, and therefore separate days should be assigned to creation.
Objection 2. Further, air and fire are nobler elements than earth and water. But one day is assigned to the distinction of water, and another to the distinction of the land. Therefore, other days ought to be devoted to the distinction of fire and air.
Objection 3. Further, fish differ from birds as much as birds differ from the beasts of the earth, whereas man differs more from other animals than all animals whatsoever differ from each other. But one day is devoted to the production of fishes, and another to that of the beast of the earth. Another day, then, ought to be assigned to the production of birds and another to that of man.
Objection 4. Further, it would seem, on the other hand, that some of these days are superfluous. Light, for instance, stands to the luminaries in the relation of accident to subject. But the subject is produced at the same time as the accident proper to it. The light and the luminaries, therefore, ought not to have been produced on different days.
Objection 5. Further, these days are devoted to the first instituting of the world. But as on the seventh day nothing was instituted, that day ought not to be enumerated with the others.
I answer that, The reason of the distinction of these days is made clear by what has been said above (Question 70, Article 1), namely, that the parts of the world had first to be distinguished, and then each part adorned and filled, as it were, by the beings that inhabit it. Now the parts into which the corporeal creation is divided are three, according to some holy writers, these parts being the heaven, or highest part, the water, or middle part, and the earth, or the lowest part. Thus the Pythagoreans teach that perfection consists in three things, the beginning, the middle, and the end. The first part, then, is distinguished on the first day, and adorned on the fourth, the middle part distinguished on the middle day, and adorned on the fifth, and the third part distinguished on the third day, and adorned on the sixth. But Augustine, while agreeing with the above writers as to the last three days, differs as to the first three, for, according to him, spiritual creatures are formed on the first day, and corporeal on the two others, the higher bodies being formed on the first these two days, and the lower on the second. Thus, then, the perfection of the Divine works corresponds to the perfection of the number six, which is the sum of its aliquot parts, one, two, three; since one day is assigned to the forming of spiritual creatures, two to that of corporeal creatures, and three to the work of adornment.
Reply to Objection 1. According to Augustine, the work of creation belongs to the production of formless matter, and of the formless spiritual nature, both of which are outside of time, as he himself says (Confess. xii, 12). Thus, then, the creation of either is set down before there was any day. But it may also be said, following other holy writers, that the works of distinction and adornment imply certain changes in the creature which are measurable by time; whereas the work of creation lies only in the Divine act producing the substance of beings instantaneously. For this reason, therefore, every work of distinction and adornment is said to take place "in a day," but creation "in the beginning" which denotes something indivisible.
Reply to Objection 2. Fire and air, as not distinctly known by the unlettered, are not expressly named by Moses among the parts of the world, but reckoned with the intermediate part, or water, especially as regards the lowest part of the air; or with the heaven, to which the higher region of air approaches, as Augustine says (Gen. ad lit. ii, 13).
Reply to Objection 3. The production of animals is recorded with reference to their adorning the various parts of the world, and therefore the days of their production are separated or united according as the animals adorn the same parts of the world, or different parts.
Reply to Objection 4. The nature of light, as existing in a subject, was made on the first day; and the making of the luminaries on the fourth day does not mean that their substance was produced anew, but that they then received a form that they had not before, as said above (70, 1 ad 2).
Reply to Objection 5. According to Augustine (Gen. ad lit. iv, 15), after all that has been recorded that is assigned to the six days, something distinct is attributed to the seventh--namely, that on it God rested in Himself from His works: and for this reason it was right that the seventh day should be mentioned after the six. It may also be said, with the other writers, that the world entered on the seventh day upon a new state, in that nothing new was to be added to it, and that therefore the seventh day is mentioned after the six, from its being devoted to cessation from work.
Source: Summa Theologica (New Advent)