The Feast of Corpus Christi, celebrated in the traditional calendar last Thursday, June 4th, is the day the Catholic Church celebrates in a special way the Most Holy Body and Blood of Our Lord.
Before the 13th century, the Church had no singular feast day exclusively devoted to the Blessed Sacrament. In the early 1200s, a Belgian Norbertine Canoness, Saint Juliana of Liege, continually received visions of a brilliant moon that had one obscured dark spot on its surface, symbolic of the Church’s lack of a feast day devoted to the Blessed Sacrament. Ultimately Our Lord Himself appeared to Saint Juliana, expressing to her that the liturgical year would be incomplete without a feast devoted to the Blessed Sacrament, and urged that a feast be established to combat the heresy and irreverence shown towards His Body and Blood that was rife during this time period.
After keeping this a secret for twenty years, St. Juliana told her confessor about her mystical experiences, and with her leave he consulted others in the church, most notably Father Jacques de Pantaleon, the French priest and Archdeacon of Liege who would eventually become Pope Urban IV.
In 1264, Pope Urban IV issued the Papal Bull Transiturus de hoc mundo (“As he was about to pass on from this world;” for those versed in Latin, the text of the Bull can be found on page 705 of this link). This Bull officially declared the Feast of Corpus Christi a regular feast in the Roman Calendar. Saint Thomas Aquinas contributed much to the establishment of this feast, as he wrote some of the the Mass’s liturgical text, as well as multiple Eucharistic hymns for the feast, including Pange Lingua and Adoro Te Devote.
The Eucharistic processions that are common with this feast day serve as a public profession of Jesus Christ’s presence in the Most Blessed Sacrament.
For more information on the history of the Feast of Corpus Christi, including the Eucharistic Miracle of Bolsena and Orvieto which may have influenced Pope Urban IV’s decision to implement the feast day, please see this link.