From the Directory on Popular Piety and the Liturgy:
160. The Solemnity of the Body and Blood of Christ is observed on the Thursday following on the solemnity of the Most Blessed Trinity. This feast is both a doctrinal and cultic response to heretical teaching on the mystery of the real presence of Christ in the Eucharist, and the apogee of an ardent devotional movement concentrated on the Sacrament of the Altar. It was extended to the entire Latin Church by Urban IV in 1264.
Popular piety encouraged the process that led to the institution of the feast of Corpus Christi, which reciprocally inspired the development of new forms of Eucharistic piety among the people of God.
For centuries, the celebration of Corpus Christi remained the principal point of popular piety's concentration on the Eucharist. In the sixteenth and seventeenth centuries, faith, in reaction to various forms of protestantism, and culture (art, folklore and literature) coalesced in developing lively and significant expressions Eucharistic devotion in popular piety.
161. Eucharistic devotion, which is so deeply rooted in the Christian faithful, must integrate two basic principles:
* the supreme reference point for Eucharistic devotion is the Lord's Passover; the Pasch as understood by the Fathers, is the feast of Easter, while the Eucharist is before all else the celebration of Paschal Mystery or of the Passion, Death and Resurrection of Christ;
* all forms of Eucharisit devotion must have an intrinsic reference to the Eucharistic Sacrifice, or dispose the faithful for its celebration, or prolong the worship which is essential to that Sacrifice.
Hence, the Rituale Romanum states "The faithful, when worshipping Christ present in the Sacrament of the Altar, should recall that this presence comes from the Sacrifice of the Eucharist, and tends towards sacramental and spiritual communion"(169).
162. The Corpus Christi procession represents the typical form of an Eucharistic procession. It is a prolongation of the celebration of the Eucharist: immediately after Mass, the Sacred Host, consecrated during the Mass, is borne out of the Church for the Christian faithful "to make public profession of faith and worship of the Most Blessed Sacrament"(170).
The faithful understand and appreciate the values inherent in the procession: they are aware of being "the People of God", journeying with the Lord, and proclaiming faith in him who has become truly "God-amongst-us".
It is necessary however to ensure that the norms governing processions be observed(171), especially those ensuring respect for the dignity and reverence of the Blessed Sacrament(172). It is also necessary to ensure that the typical elements of popular piety accompanying the procession, such as the decoration of the streets and windows with flowers and the hymns and prayers used during the procession, truly "lead all to manifest their faith in Christ, and to give praise to the Lord"(173), and exclude any forms of competition.
163. The Eucharistic procession is normally concluded by a blessing with the Blessed Sacrament. In the specific case of the Corpus Christi procession, the solemn blessing with the Blessed Sacrament concludes the entire celebration: the usual blessing by the priest is replaced by the blessing with the Blessed Sacrament.
It is important that the faithful understand that this blessing is not an independent form of Eucharistic piety, but the end of a prolonged act of worship. Hence, liturgical norms prohibit "exposition of the Blessed Sacrament for the purpose of giving the blessing"(174).