The Feast of Corpus Christi is celebrated on the Thursday or Sunday
following Trinity Sunday the second Sunday of Pentecost.This appeared on the Catholic Calendar due
to the petitions of the thirteenth-century Augustinian nun Juliana of
Liège. She claimed that God had been instructing her to
establish a feast day for the Eucharist and later in life she petitioned the
learned Dominican Hugh of St-Cher, Jacques Pantaléon (Archdeacon of
Liège and later Pope Urban IV) and Robert de Thorete, Bishop of Liège.
At that time bishops could order feasts in their dioceses, so in 1246
Bishop Robert convened a synod and ordered a celebration of Corpus
Christi to be held each year thereafter. The decree is preserved in
Anton Joseph Binterim’s Vorzüglichsten Denkwürdigkeiten der
Christkatholischen Kirche, together with parts of the first liturgy
written for the occasion.

The celebration of Corpus Christi only
became widespread after both Juliana and Bishop Robert had died. In 1263
Pope Urban IV investigated claims of a miracle in which blood had
issued from a host. In 1264 he issued the papal bull Transiturus in
which Corpus Christi was made a feast day. A new liturgy for the
celebration was written by Thomas Aquinas.