Psalms 135:8

2. The Psalm hath this title, that is, this inscription: “A Psalm of David himself, on the fourth day of the week.” This Psalm is about to teach patience in the sufferings of the righteous: it enjoineth patience against the prosperity of the wicked, and buildeth up patience. This is the drift of the whole of it, from beginning to end. Wherefore then hath it such a title, “on the fourth of the week”? The first of the week is the Lord’s day: the second, is the second week-day, which people of the world call the Moon’s day: the third, is the third weekday, which they term Mars’ day. The fourth of the Sabbaths therefore is the fourth week-day, which by Pagans is styled Mercury’s day, and also by many Christians; but I would not call it so: and I wish they would change for the better, and cease to do so; for they have a phrase of their own, which they may use. …

5. But what meaneth, “who alone doeth great wonders”? Is it because many wonderful things He hath done by means of angels and men? Some wonderful things there are which God doeth alone, and these he enumerates, saying, “who by His wisdom made the heavens” ( ver. 5 ), “who stretched out the earth above the waters” ( ver. 6 ), “who alone made great lights” ( ver. 7 ). For this reason did he add “alone” in this verse also, because the other wonders which he is about to tell of, God did by means of man. For having said, “who alone made great lights,” he goes on to explain what these are, “the sun to rule the day” ( ver. 8 ), “the moon and stars to govern the night” ( ver. 9 ); then he begins to tell the wonders which He did by means of angels and men: “who smote Egypt with their first-born” ( ver. 10 ), and the rest. …

“Let them be for days” says Scripture, not to produce them but to rule them; because day and night are older than the creation of the luminaries and it is this that the psalm declares to us. “The sun to rule by day…the moon and stars to rule by night.” How does the sun rule by day? Because carrying everywhere light with it, it is no sooner risen above the horizon than it drives away darkness and brings us day. Thus we might, without self deception, define day as air lighted by the sun, or as the space of time that the sun passes in our hemisphere. The functions of the sun and moon serve further to mark years. The moon, after having twelve times run her course, forms a year which sometimes needs an intercalary month to make it exactly agree with the seasons. Such was formerly the year of the Hebrews and of the early Greeks. …