Genesis 1:5

He did not say "night and day," but "one day," with reference to the name of the light. He did not say the "first day; "for if he had said the "first" day, he would also have had to say that the "second" day was made. But it was right to speak not of the "first day," but of "one day," in order that by saying "one," he might show that it returns on its orbit and, while it remains one, makes up the week.

So let no one think that there is anything allegorical in the works of the six days. No one can rightly say that the things pertaining to these days were symbolic, nor can one say that they were meaningless names or that other things were symbolized for us by their names. Rather, let us know in just what manner heaven and earth were created in the beginning. They were truly heaven and earth. There was no other thing signified by the names “heaven” and “earth.” The rest of the works and things made that followed were not meaningless significations either, for the substances of their natures correspond to what their names signify.

Heaven, earth, fire, wind and water were created from nothing as Scripture bears witness. But light, which came to be on the first day along with the rest of the things that came to be afterwards, came to be from something. For when these other things came to be from nothing, Moses said, "God created heaven and earth." Although it is not written concerning fire, water and wind that they were created, neither is it written that they were made. Therefore, they came to be from nothing just as heaven and earth came to be from nothing.
After God began to make [things] from something, Moses wrote, "God said, 'Let there be'" light, and so on. Even though Moses did say, "God created the great serpents," still "let the waters swarm with swarming things" had been [ said ] prior to that. Therefore those five created things were created from nothing and everything else was made from those [ five ] things that came to be from nothing.
Fire was also created on the first day, although it is not written down that it was created, because it was in another element. It did not have its own existence, for it was created together with that thing in which it was. It is not possible that a thing which does not exist of itself can precede that thing which is the cause of its existence. That [ fire ] is in the earth, nature bears witness, but that it was not created together with the earth, scripture affirms, when it says, "In the beginning God created heaven and earth." Fire then, since it does not exist of itself, remains with the earth, even if the wind and the clouds have been commanded at every moment to bring forth fire from their wombs along with the wind and the clouds.
Darkness, too, is neither a self-subsistent being nor a created thing, but is a shadow, as scripture makes clear. It was created neither before heaven nor after the clouds, for it was with the clouds and was brought forth from the clouds. [ Darkness ] too exists in another [ thing ], for it has no substance of its own. When that in which it exists vanishes, the darkness likewise vanishes with it. For whatever comes to an end along with another thing when it vanishes is without its own existence, because that other thing is the cause of its existence.
So, how could darkness, whose existence is due to the clouds and to the firmament and not to the first light or to the sun, exist of itself? It is [ a thing ] which one thing, by its cover, brings forth and another, by its brightness, destroys. If one thing creates it and causes it to become something while another thing turns it back into nothing, how can it be a self-subsistent being? The clouds and the firmament, which were created at the beginning, bring it forth and the light that was created on the first day brings it to an end. If a created thing creates it and another created thing destroys it, and henceforth, one thing, at one moment, brings it into visibility and another, at that very moment itself turns back into nothing, turns it back into nothing, it is by compulsion that [ one thing ] causes it to begin and [ another thing ] causes it to go away. If created things cause it to come into existence and also cause it to vanish then it is a creation of creatures. [ The darkness then ] is but a shadow of the firmament and it is capable of vanishing in the presence of another thing, for it can be destroyed before the sun. Some teachings posit that this [ darkness ], which is at all times subject to created things, is an adversary of creatures, and they make that thing which has no substance of its own a self-existent being.

In fact, there did exist something, as it seems, even before this world which our mind can attain by contemplation but which has been left uninvestigated because it is not adapted to those who are beginners and as yet infants in understanding. This was a certain condition older than the birth of the world and proper to the supramundane powers, one beyond time, everlasting, without beginning or end. In it the Creator and Producer of all things perfected the works of his art, a spiritual light befitting the blessedness of those who love the Lord, rational and invisible natures, and the whole orderly arrangement of spiritual creatures which surpass our understanding and of which it is impossible even to discover the names. These fill completely the essence of the invisible world.

Since the birth of the sun, the light that it diffuses in the air, when shining on our hemisphere, is day; and the shadow produced by its disappearance is night. But at that time it was not after the movement of the sun, but following this primitive light spread abroad in the air or withdrawn in a measure determined by God, that day came and was followed by night.

Evening is then the boundary common to day and night; and in the same way morning constitutes the approach of night to day. It was to give day the privileges of seniority that Scripture put the end of the first day before that of the first night, because night follows day: for, before the creation of light, the world was not in night, but in darkness. It is the opposite of day which was called night, and it did not receive its name until after day. Thus were created the evening and the morning. Scripture means the space of a day and a night, and afterwards no more says day and night, but calls them both under the name of the more important: a custom which you will find throughout Scripture. Everywhere the measure of time is counted by days, without mention of nights. The days of our years, says the Psalmist. Few and evil have the days of the years of my life been, Genesis 47:9 said Jacob, and elsewhere all the days of my life. Thus under the form of history the law is laid down for what is to follow. And the evening and the morning were one day. Why does Scripture say one day the first day? Before speaking to us of the second, the third, and the fourth days, would it not have been more natural to call that one the first which began the series? If it therefore says one day, it is from a wish to determine the measure of day and night, and to combine the time that they contain. Now twenty-four hours fill up the space of one day— we mean of a day and of a night; and if, at the time of the solstices, they have not both an equal length, the time marked by Scripture does not the less circumscribe their duration. It is as though it said: twenty-four hours measure the space of a day, or that, in reality a day is the time that the heavens starting from one point take to return there. Thus, every time that, in the revolution of the sun, evening and morning occupy the world, their periodical succession never exceeds the space of one day. But must we believe in a mysterious reason for this? God who made the nature of time measured it out and determined it by intervals of days; and, wishing to give it a week as a measure, he ordered the week to revolve from period to period upon itself, to count the movement of time, forming the week of one day revolving seven times upon itself: a proper circle begins and ends with itself. Such is also the character of eternity, to revolve upon itself and to end nowhere. If then the beginning of time is called one day rather than the first day, it is because Scripture wishes to establish its relationship with eternity. It was, in reality, fit and natural to call one the day whose character is to be one wholly separated and isolated from all the others. If Scripture speaks to us of many ages, saying everywhere, age of age, and ages of ages, we do not see it enumerate them as first, second, and third. It follows that we are hereby shown not so much limits, ends and succession of ages, as distinctions between various states and modes of action. The day of the Lord, Scripture says, is great and very terrible, Joel 2:11 and elsewhere Woe unto you that desire the day of the Lord: to what end is it for you? The day of the Lord is darkness and not light. Amos 5:18 A day of darkness for those who are worthy of darkness. No; this day without evening, without succession and without end is not unknown to Scripture, and it is the day that the Psalmist calls the eighth day, because it is outside this time of weeks. Thus whether you call it day, or whether you call it eternity, you express the same idea. Give this state the name of day; there are not several, but only one. If you call it eternity still it is unique and not manifold. Thus it is in order that you may carry your thoughts forward towards a future life, that Scripture marks by the word one the day which is the type of eternity, the first fruits of days, the contemporary of light, the holy Lord's day honoured by the Resurrection of our Lord. And the evening and the morning were one day.

But, while I am conversing with you about the first evening of the world, evening takes me by surprise, and puts an end to my discourse. May the Father of the true light, Who has adorned day with celestial light, Who has made the fire to shine which illuminates us during the night, Who reserves for us in the peace of a future age a spiritual and everlasting light, enlighten your hearts in the knowledge of truth, keep you from stumbling, and grant that you may walk honestly as in the day. Romans 13:13 Thus shall you shine as the sun in the midst of the glory of the saints, and I shall glory in you in the day of Christ, to Whom belong all glory and power for ever and ever. Amen.

Now, henceforth, after the creation of the sun, it is day when the air is illuminated by the sun shining on the hemisphere above the earth, and night is the darkness of the earth when the sun is hidden. Yet it was not at that time according to solar motion, but it was when that first created light was diffused and again drawn in according to the measure ordained by God, that day came and night succeeded.

The angels, dominions and powers, although they began to exist at some time, were already in existence when the [visible] world was created.

“And God divided the light and the darkness, and God called the light day and he called the darkness night.” It did not say here “God made the darkness,” because darkness is merely the absence of light. Yet God made a division between light and darkness. So too we make a sound by crying out, and we make a silence by not making a sound, because silence is the cessation of sound. Still in some sense we distinguish between sound and silence and call the one sound and the other silence.… “He called the light day, and he called the darkness night” was said in the sense that he made them to be called, because he separated and ordered all things so that they could be distinguished and receive names.

And there was evening and there was morning, one day. And there was evening as the light gradually waned following the completion of the period of daytime length, and as the lower parts of the world emerged, which now habitually happens by the circuit of the sun throughout the night; and there was morning as the same light gradually returned over the earth and initiated another day; and thus one day was completed, namely of twenty-four hours, for the commendation of which the Scripture vigilantly admonishes us so that we might learn that the light which was made illuminated the lower parts of the earth by its setting. For if this did not happen, but rather as evening came, the whole light perished gradually, and gradually returned with the morning and rose again, it would not call it a perfect day in the morning of the next day but in the evening of the first day. Hence, it also preferred to say evening and morning, rather than night and day, to imply that the action of the original light was by circuit, which now indeed happens by the circuit of the sun night and day; beyond this only, that after the stars were created, night too is suffused with its own light, although lesser than that of the day. However, during those first three days, the night remained entirely gloomy and obscure. It was completely fitting that the day beginning from the light should be extended into the morning of the following day, so that it might be intimated that the works of Him who is the true light, and in whom there are no shadows, begin from the light and are completed in the light.

And He called the light Day, and the darkness He called Night. This was said for our understanding; for in what language did God call the light Day and the darkness Night; whether in Hebrew, or Greek, or some other language! And so for everything else that He named, it can be asked in what language He named them; but with God, there is pure understanding without noise and diversity of tongues. However, it is said He "called" because He made them to be called, as He distinguished and ordered everything so that Days could be seen and names given. For we say: That householder built this house, meaning he had it built, and many such examples are found throughout the books of divine Scriptures.

"And there was evening and morning one day": According to Basil (Hom. ii in Hexaem.), the entire period takes its name, as is customary, from its more important part, the day. And instance of this is found in the words of Jacob, "The days of my pilgrimage," where night is not mentioned at all. But the evening and the morning are mentioned as being the ends of the day, since day begins with morning and ends with evening, or because evening denotes the beginning of night, and morning the beginning of day. It seems fitting, also, that where the first distinction of creatures is described, divisions of time should be denoted only by what marks their beginning. And the reason for mentioning the evening first is that as the evening ends the day, which begins with the light, the termination of the light at evening precedes the termination of the darkness, which ends with the morning. But Chrysostom's explanation is that thereby it is intended to show that the natural day does not end with the evening, but with the morning (Hom. v in Gen.).
Or else it can be said, as Augustine puts it (Gen. ad lit. iv, 23), that there is nothing to prevent us from calling something light in comparison with one thing, and darkness with respect to another. In the same way the life of the faithful and the just is called light in comparison with the wicked, according to Ephesians 5:8: "You were heretofore darkness; but now, light in the Lord": yet this very life of the faithful, when set in contrast to the life of glory, is termed darkness, according to 2 Peter 1:19: "You have the firm prophetic word, whereunto you do well to attend, as to a light that shineth in a dark place." So the angel's knowledge by which he knows things in their own nature, is day in comparison with ignorance or error; yet it is dark in comparison with the vision of the Word.