From the Catechism of the Catholic Church
At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, become Christ's Body and Blood. Faithful to the Lord's command the Church continues to do, in his memory and until his glorious return, what he did on the eve of his Passion: "He took bread. . . ." "He took the cup filled with wine. . . ." The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ; they continue also to signify the goodness of creation. Thus in the Offertory we give thanks to the Creator for bread and wine, fruit of the "work of human hands," but above all as "fruit of the earth" and "of the vine" - gifts of the Creator. The Church sees in the gesture of the king-priest Melchizedek, who "brought out bread and wine," a prefiguring of her own offering.
2041 The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the very necessary minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbour:
2042 The first precept ("You shall attend Mass on Sundays and holy days of obligation and rest from servile labour") requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honouring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days. The second precept ("You shall confess your sins at least once a year") ensures preparation for the Eucharist by the reception of the sacrament of reconciliation, which continues Baptism's work of conversion and forgiveness. The third precept ("You shall receive the sacrament of the Eucharist at least during the Easter season") guarantees as a minimum the reception of the Lord's Body and Blood in connection with the Paschal feasts, the origin and centre of the Christian liturgy.