Objection 1. It would seem that all these days are one day. For it is written (Genesis 2:4-5): "These are the generations of the heaven and the earth, when they were created, in the day that the Lord... made the heaven and the earth, and every plant of the field, before it sprung up in the earth." Therefore the day in which God made "the heaven and the earth, and every plant of the field," is one and the same day. But He made the heaven and the earth on the first day, or rather before there was any day, but the plant of the field He made on the third day. Therefore the first and third days are but one day, and for a like reason all the rest.
Objection 2. Further, it is said (Sirach 18:1): "He that liveth for ever, created all things together." But this would not be the case if the days of these works were more than one. Therefore they are not many but one only.
Objection 3. Further, on the seventh day God ceased from all new works. If, then, the seventh day is distinct from the other days, it follows that He did not make that day; which is not admissible.
Objection 4. Further, the entire work ascribed to one day God perfected in an instant, for with each work are the words (God) "said.... and it was... done." If, then, He had kept back His next work to another day, it would follow that for the remainder of a day He would have ceased from working and left it vacant, which would be superfluous. The day, therefore, of the preceding work is one with the day of the work that follows.
On the contrary, It is written (Genesis 1), "The evening and the morning were the second day... the third day," and so on. But where there is a second and third there are more than one. There was not, therefore, only one day.
I answer that, On this question Augustine differs from other expositors. His opinion is that all the days that are called seven, are one day represented in a sevenfold aspect (Gen. ad lit. iv, 22; De Civ. Dei xi, 9; Ad Orosium xxvi); while others consider there were seven distinct days, not one only. Now, these two opinions, taken as explaining the literal text of Genesis, are certainly widely different. For Augustine understands by the word "day," the knowledge in the mind of the angels, and hence, according to him, the first day denotes their knowledge of the first of the Divine works, the second day their knowledge of the second work, and similarly with the rest. Thus, then, each work is said to have been wrought in some one of these days, inasmuch as God wrought in some one of these days, inasmuch as God wrought nothing in the universe without impressing the knowledge thereof on the angelic mind; which can know many things at the same time, especially in the Word, in Whom all angelic knowledge is perfected and terminated. So the distinction of days denotes the natural order of the things known, and not a succession in the knowledge acquired, or in the things produced. Moreover, angelic knowledge is appropriately called "day," since light, the cause of day, is to be found in spiritual things, as Augustine observes (Gen. ad lit. iv, 28). In the opinion of the others, however, the days signify a succession both in time, and in the things produced.
If, however, these two explanations are looked at as referring to the mode of production, they will be found not greatly to differ, if the diversity of opinion existing on two points, as already shown (67, 1; 69, 1), between Augustine and other writers is taken into account.
First, because Augustine takes the earth and the water as first created, to signify matter totally without form; but the making of the firmament, the gathering of the waters, and the appearing of dry land, to denote the impression of forms upon corporeal matter. But other holy writers take the earth and the water, as first created, to signify the elements of the universe themselves existing under the proper forms, and the works that follow to mean some sort of distinction in bodies previously existing, as also has been shown (67, 1,4; 69, 1).
Secondly, some writers hold that plants and animals were produced actually in the work of the six days; Augustine, that they were produced potentially. Now the opinion of Augustine, that the works of the six days were simultaneous, is consistent with either view of the mode of production. For the other writers agree with him that in the first production of things matter existed under the substantial form of the elements, and agree with him also that in the first instituting of the world animals and plants did not exist actually. There remains, however, a difference as to four points; since, according to the latter, there was a time, after the production of creatures, in which light did not exist, the firmament had not been formed, and the earth was still covered by the waters, nor had the heavenly bodies been formed, which is the fourth difference; which are not consistent with Augustine's explanation. In order, therefore, to be impartial, we must meet the arguments of either side.
Reply to Objection 1. On the day on which God created the heaven and the earth, He created also every plant of the field, not, indeed, actually, but "before it sprung up in the earth," that is, potentially. And this work Augustine ascribes to the third day, but other writers to the first instituting of the world.
Reply to Objection 2. God created all things together so far as regards their substance in some measure formless. But He did not create all things together, so far as regards that formation of things which lies in distinction and adornment. Hence the word "creation" is significant.
Reply to Objection 3. On the seventh day God ceased from making new things, but not from providing for their increase, and to this latter work it belongs that the first day is succeeded by other days.
Reply to Objection 4. All things were not distinguished and adorned together, not from a want of power on God's part, as requiring time in which to work, but that due order might be observed in the instituting of the world. Hence it was fitting that different days should be assigned to the different states of the world, as each succeeding work added to the world a fresh state of perfection.
Reply to Objection 5. According to Augustine, the order of days refers to the natural order of the works attributed to the days.
Source: Summa Theologica (New Advent)