Objection 1. It would seem that there is neither an evening nor a morning knowledge in the angels; because evening and morning have an admixture of darkness. But there is no darkness in the knowledge of an angel; since there is no error nor falsehood. Therefore the angelic knowledge ought not to be termed morning and evening knowledge.
Objection 2. Further, between evening and morning the night intervenes; while noonday falls between morning and evening. Consequently, if there be a morning and an evening knowledge in the angels, for the same reason it appears that there ought to be a noonday and a night knowledge.
Objection 3. Further, knowledge is diversified according to the difference of the objects known: hence the Philosopher says (De Anima iii, text. 38), "The sciences are divided just as things are." But there is a threefold existence of things: to wit, in the Word; in their own natures; and in the angelic knowledge, as Augustine observes (Gen. ad lit. ii, 8). If, therefore, a morning and an evening knowledge be admitted in the angels, because of the existence of things in the Word, and in their own nature, then there ought to be admitted a third class of knowledge, on account of the existence of things in the angelic mind.
On the contrary, Augustine (Gen. ad lit. iv, 22,31; De Civ. Dei xii, 7,20) divides the knowledge of the angels into morning and evening knowledge.
I answer that, 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.
Reply to Objection 1. Evening and morning knowledge in the angelic knowledge are not taken as compared to an admixture of darkness, but as compared to beginning and end. 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.
Reply to Objection 2. The morning and evening knowledge belong to the day, that is, to the enlightened angels, who are quite apart from the darkness, that is, from the evil spirits. 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. Consequently after "evening" there is no night, but "morning"; so that morning is the end of the preceding day, and the beginning of the following, in so far as the angels refer to God's praise their knowledge of the preceding work. Noonday is comprised under the name of day, as the middle between the two extremes. Or else the noon can be referred to their knowledge of God Himself, Who has neither beginning nor end.
Reply to Objection 3. The angels themselves are also creatures. Accordingly the existence of things in the angelic knowledge is comprised under evening knowledge, as also the existence of things in their own nature.
Source: Summa Theologica (New Advent)