7 But to us it came: there came too the solemn day, in which we ought to call to the feast with a trumpet, and separate ourselves to the Lord with thanksgiving, considering it as our own festival. For we are bound to celebrate it, not to ourselves but to the Lord; and to rejoice, not in ourselves but in the Lord, who bore our griefs and said, 'My soul is sorrowful unto death.' For the heathen, and all those who are strangers to our faith, keep feasts according to their own wills, and have no peace, since they commit evil against God. But the saints, as they live to the Lord also keep the feast to Him, saying, 'I will rejoice in Your salvation,' and, 'my soul shall be joyful in the Lord.' The commandment is common to them, 'Rejoice, you righteous, in the Lord '— so that they also may be gathered together, to sing that common and festal Psalm, 'Come, let us rejoice,' not in ourselves, but, 'in the Lord.'
Source: Letters (New Advent)