2 This Psalm is entitled, a Psalm to be sung on the Sabbath day. Lo, this day is the Sabbath, which the Jews at this period observe by a kind of bodily rest, languid and luxurious. They abstain from labours, and give themselves up to trifles; and though God ordained the Sabbath, they spend it in actions which God forbids. Our rest is from evil works, theirs from good; for it is better to plough than to dance. They abstain from good, but not from trifling, works. God proclaims to us a Sabbath. What sort of Sabbath? First consider, where it is. It is in the heart, within us; for many are idle with their limbs, while they are disturbed in conscience....That very joy in the tranquillity of our hope, is our Sabbath. This is the subject of praise and of song in this Psalm, how a Christian man is in the Sabbath of his own heart, that is, in the quiet, tranquillity, and serenity of his conscience, undisturbed; hence he tells us here, whence men are wont to be disturbed, and he teaches you to keep Sabbath in your own heart.
3....Accuse yourself, and you receive indulgence. Besides, many do not accuse Satan but their fate. My fate led me, says one when you ask him, why did you do it? Why did you sin? He replies, by my evil fate. Lest he should say, I did it; he points to God as the source of his sin: with his tongue he blasphemes. He says not this indeed openly as yet, but listen, and see that he says this. You ask of him, what is fate: and he replies, evil stars. You ask, who made, who appointed the stars; he can only answer, God. It follows, then, that whether he does so directly or indirectly, still he accuses God, and when God punishes sins, he makes God the author of his own sins. It cannot be that God punishes what He has wrought: He punishes what you do, that He may set free what He has wrought. But sometimes, setting aside everything else, they attack God directly: and when they sin, they say, God willed this; if God had not willed it, I should not have sinned. Does He warn you for this, that not only He may not be listened to, to keep you from sin, but even be accused because you sin? What then does this Psalm teach us? “It is a good thing to confess unto the Lord.” What is to confess unto the Lord? In both cases: both in your sins, because you have done them; and in your good works, confess unto the Lord, because He has done them. Then shall you “sing unto the Name of God, the Most Highest:” seeking the glory of God, not your own; His Name, not yours. For if you seek the Name of God, He also seeks your name; but if you have neglected the Name of God, He also does blot out yours....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)