The fourth precept (“You shall keep holy the holy days of obligation.") completes the Sunday observance by participation in the principal liturgical feasts which honor the mysteries of the Lord, the Virgin Mary, and the saints.
The fifth precept (“You shall observe the prescribed days of fasting and abstinence.") ensures the times of ascesis and penance which prepare us for the liturgical feasts; they help us acquire mastery over our instincts and freedom of heart.
The faithful also have the duty of providing for the material needs of the Church, each according to his abilities.
Source: Catechism of the Catholic Church (Vatican.va)