Day as Knowledge and Night as Ignorance
And first of all, indeed, light was made by the word of God, and God, we read, separated it from the darkness, and called the light Day, and the darkness Night; …
For the knowledge of the creature is, in comparison of the knowledge of the Creator, but a twilight; and so it dawns and breaks into morning when the creature is drawn to the praise and love of the Creator; and night never falls when the Creator is not forsaken through love of the creature. In fine, Scripture, when it would recount those days in order, never mentions the word night. It never says, “Night was,” but “The evening and the morning were the first day.” So of the second and the rest. And, indeed, the knowledge of created things contemplated by themselves is, so to speak, more colorless than when they are seen in the wisdom of God, as in the art by which they were made. Therefore evening is a more suitable figure than night; and yet, as I said, morning returns when the creature returns to the praise and love of the Creator.
When it does so in the knowledge of itself, that is the first day; when in the knowledge of the firmament, which is the name given to the sky between the waters above and those beneath, that is the second day; when in the knowledge of the earth, and the sea, and all things that grow out of the earth, that is the third day; when in the knowledge of the greater and less luminaries, and all the stars, that is the fourth day; when in the knowledge of all animals that swim in the waters and that fly in the air, that is the fifth day; when in the knowledge of all animals that live on the earth, and of man himself, that is the sixth day.
— City of God. XI.7