2 The Psalm has 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 enjoins patience against the prosperity of the wicked, and builds up patience. This is the drift of the whole of it, from beginning to end. Wherefore then has 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. For these terms are not of universal use: many nations have severally different names for them: so that the mode of speech used by the Church better beseems the mouth of a Christian. Yet if custom has induced any person to utter that with his tongue which his heart does disapprove, let him remember, that all those whose names the stars bear were men, and that the stars did not commence their existence in the sky, when those men began theirs, but were there long before; but on account of some mortal services rendered unto mortals, those men in their own times, because they had great power, and were eminent in this life, since they were beloved by men, not on account of eternal life, but of temporal services, received divine honours.
For then men of the old world, in being deceived and wishing to deceive, pointed to the stars in heaven, to flatter those who had done them any good service in their affection for this life, saying, that that was the star of such a man, this of another; while the man who had not beheld them before, so as to see that those stars were there before the birth of the man, were deceived into a belief: and thus this vain opinion was conceived. This erroneous opinion the devil strengthened, Christ overthrew.
According to our mode of speech, then, the fourth of the week is taken for the fourth day from the Lord's day. Attend, therefore, beloved, to what this title means. Here is a great mystery, and a truly hidden one....Let us therefore recall from the holy Scripture in Genesis, what was created on the first day; we find light: what was created on the second day; we find the firmament, which God called heaven: what was created on the third day; we find the form of earth and sea, and their separation, that all the gathering together of the waters was called sea, and all that was dry, the earth.
On the fourth day, the Lord made the lights in heaven: “The sun to rule the day: the moon and stars to govern the night:” this was the work of the fourth day. What then is the reason that the Psalm has taken its title from the fourth day: the Psalm in which patience is enjoined against the prosperity of the wicked, and the sufferings of the good. Thou findest the Apostle Paul speaking. “Do all things without murmurings and disputings: that you may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom you shine as lights in the world, holding forth the word of life.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)