Why the first day is called one, not first

And the evening star is the most beautiful of the stars, not because its parts are in proportion to one another, but because its radiance falls on the eyes in a way that is painless and pleasant. Then again, God's judgment about the beautiful was made not only with a view to what delights the sight, but also with foresight of the benefit that would come from it later. For there were as yet no eyes to judge the beauty in light. And God separated between the light and between the darkness (Genesis 1:4). That is, God made their natures unmixed and opposed to one another in contrariety. For he set them apart and divided them by the greatest distance.

And God called the light day, and the darkness he called night (Genesis 1:5). Now, after the creation of the sun, day is the air lit by the sun when it shines in the hemisphere above the earth, and night is the shadow of the earth that occurs when the sun is hidden. But then it was not according to the sun's movement; rather, as that firstborn light was poured forth and again contracted according to the measure fixed by God, day came to be, and night followed in turn. And there was evening, and there was morning, one day (Genesis 1:5). Evening, then, is the common boundary of day and night; and morning likewise is the neighborhood of night to day. So in order to give the privileges of birth to the day, he first named the end of the day, and then the end of the night, since night follows after day. For the state of the world before the creation of light was not night, but darkness; what was set in contrast to the day was named night, and it received this newer name after the day. So there was evening, and there was morning. He means the day-night. And he no longer named them day and night, but assigned the whole name to the prevailing one. You would find this same custom throughout all Scripture: in the measuring of time, days are counted, and not nights along with the days. The days of our years, says the psalmist (Psalm 89:10 LXX, Hebrew 90:10). And again Jacob: The days of my life are few and evil (Genesis 47:9). And again: All the days of my life (Psalm 26:4 LXX, Hebrew 27:4). So what has now been handed down in the form of history is legislation for what follows. And there was evening, and there was morning, one day (Genesis 1:5). Why did he not say first, but one? And yet it would have been more consistent for the one who was about to bring in a second, third, and fourth day to call the day that begins the sequence first. But he said one, either defining the boundary of the measure of day and night and joining together the time of the day-night, since the twenty-four hours complete the interval of one day—night being clearly included along with the day—so that even if at the turnings of the sun one of them happens to exceed the other, the intervals of both are nevertheless always circumscribed within the appointed time. It is as if he were saying: the measure of twenty-four hours is the interval of one day; or, the return of the heaven from the same point to the same point again happens in one day. So however often evening and morning overtake the world according to the sun's circuit, the period is completed not in more time, but in the interval of one day.