Augustine of Hippo

"Day unto day utters word" [Psalm 19:2]. To the spiritual the Spirit gives out the fullness of the unchangeable Wisdom of God, the Word which in the beginning is God with God. [John 1:1] "And night unto night announces knowledge." And to the fleshly, as to those afar off, the mortality of the flesh, by conveying faith, announces future knowledge.

Exposition on Psalm 19 ↗

Thomas Aquinas

"Day." Here he excludes the impediment to teaching, and he excludes a threefold impediment. Sometimes teaching is impeded so that it cannot be learned because of time; sometimes because of the variety of languages; sometimes because of diversity of places. The first impediment concerns nighttime, for it is not a time for reading, since it is a time for resting; and according to different times it is fitting to teach and to learn. …

Exposition on the Psalms of David ↗

Bonaventure

In relation to unity, there are two testaments, the one giving birth into slavery and, the other, into freedom; the one of fear and the other of love, the one literal and the other spiritual, the one figurative and the other actual: and so these two times are distinguished as are the night and the day. Hence, in the Psalm: "Day pours out the word to day, and night to night imparts knowledge." During that night, the Law was like the moon, and according to Gregory the Fathers were like the stars. …

Collations on the Hexaemeron, Collation 15 ↗