From the Office of Readings: A Commentary on Joel, by St. Jerome
Return to me with all your heart and show a spirit of repentance with fasting, weeping and mourning; so that while you fast now, later you may be satisfied, while you weep now, later you may laugh, while you mourn now, you may some day enjoy consolation.
It is customary for those in sorrow or adversity to tear their garments. The gospel records that the high priest did this to exaggerate the charge against our Lord and Saviour; and we read that Paul and Barnabas did so when they heard words of blasphemy. I bid you not to tear your garments but rather to rend your hearts which are laden with sin. Like wine skins, unless they have been cut open, they will burst of their own accord. After you have done this, return to the Lord your God, from whom you had been alienated by your sins. Do not despair of his mercy, no matter how great your sins, for great mercy will take away great sins.
For the Lord is gracious and merciful and prefers the conversion of a sinner rather than his death. Patient and generous in his mercy, he does not give in to human impatience but is willing to wait a long time for our repentance. So extraordinary is the Lord’s mercy in the face of evil, that if we do penance for our sins, he regrets his own threat and does not carry out against us the sanctions he had threatened. So by the changing of our attitude, he himself is changed. But in this passage we should interpret “evil” to mean, not the opposite of virtue, but affliction, as we read in another place: Sufficient for the day are its own evils. And, again: If there is evil in the city, God did not create it.