with fasting, and weeping, and mourning;
Rend your hearts, not your garments,
and return to the LORD, your God.
-- Jl 2:12-18
Giving alms to the poor is a witness to fraternal charity: it is also a work of justice pleasing to God.
-- CCC 2462
"Pray constantly . . . always and for everything giving thanks in the name of our Lord Jesus Christ to God the Father." St. Paul adds, "Pray at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance making supplication for all the saints." For "we have not been commanded to work, to keep watch and to fast constantly, but it has been laid down that we are to pray without ceasing." This tireless fervor can come only from love. Against our dullness and laziness, the battle of prayer is that of humble, trusting, and persevering love. This love opens our hearts to three enlightening and life-giving facts of faith about prayer.
-- CCC 2742
The fourth precept ("You shall observe the days of fasting and abstinence established by the Church") ensures the times of ascesis and penance which prepare us for the liturgical feasts and help us acquire mastery over our instincts and freedom of heart.
-- CCC 2043
Jesus' call to conversion and penance, like that of the prophets before him, does not aim first at outward works, "sackcloth and ashes," fasting and mortification, but at the conversion of the heart, interior conversion. Without this, such penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance.
-- CCC 1430