To understand this idea, you have to go back to Augustine's musings on the text of Genesis 1. Augustine noted that one might understand "In the beginning God created the heavens and the earth" in more than one way, depending on how one understood 'the heavens and the earth'. Should this be understood as referring to both spiritual and corporeal creatures, or just corporeal creatures? He then discussed how one would go about interpreting the text if one took the first view. It's important to note two things: Augustine is discussing the literal meaning of the text, not any allegorical reading; and most of his discussion is very hypothetical and theoretical, because he is very cautious about taking a definitive stand on such a question. He explicitly contrasts the suggestion with interpretations that, for instance, would take the creation-story to be an allegory about sin and renewal; and he insists that in obscure matters like the creation we should not be overly fond of our own preferred opinions.
On Augustine's suggested interpretation, the days of creation mentioned in Genesis 1 are actually categories or stages of angelic knowledge. From "let there be light" where, so to speak, the angelic lights are turned on and angels begin to exist and to know God and the world, through the rest of the days, we get the creation from an angelic viewpoint. In this context, he suggests that "And evening and morning were the nth day" can be seen as a sort of cycle of angelic knowledge, or at least two aspects of angelic knowing -- evening knowledge and morning knowledge. As Aquinas summarizes it (ST 1.58.6):
The expression "morning" and "evening" knowledge was devised by Augustine; who interprets the six days wherein God made all things, not as ordinary days measured by the solar circuit, since the sun was only made on the fourth day, but as one day, namely, the day of angelic knowledge as directed to six classes of things. As in the ordinary day, morning is the beginning, and evening the close of day, so, their knowledge of the primordial being of things is called morning knowledge; and this is according as things exist in the Word. But their knowledge of the very being of the thing created, as it stands in its own nature, is termed evening knowledge; because the being of things flows from the Word, as from a kind of primordial principle; and this flow is terminated in the being which they have in themselves.
So in evening knowledge, the angels know created things as they are in themselves (including, Aquinas insists a bit later, themselves). In morning knowledge, however, the angel has (as it were) turned from the thing itself to know it as it exists in the Divine Word through which it is created. This is a deeper and less obscure knowledge for it is to know not only the thing itself but the thing itself insofar as it is related to its cause, and, what is more, insofar as it is related to its ultimate and most enlightening cause. Aquinas sums it up quite nicely by saying in reply to an objection, "The good angels, while knowing the creature, do not adhere to it, for that would be to turn to darkness and to night; but they refer this back to the praise of God, in Whom, as in their principle, they know all things." Starting with evening knowledge, the angel moves into morning knowledge; the morning knowledge is the culmination of one cycle of knowledge and the preparation for another.
There is a bit more complication here than might be immediately obvious. Angels, unlike us, don't know things by sensing material objects and drawing conclusions from them. Instead, their natural knowledge is innate (or infused by God). So there's a sense in which the turning of the angelic day from evening to morning is a turning from themselves to God. Thus knowledge for the angels is not merely a bare acquaintance with things but a submission to Truth Itself -- as it should be for all of us. This is why Aquinas makes the qualification 'good angels' -- the fall of the wicked angels is a failure to move from the knowledge of things to the knowledge of God. The demonic life is a night without morning, because they do not attempt to look at what they know in the light of the rising Sun.
