The Feast of Corpus Christi
The Feast of Corpus Christi ("the Body of Christ") celebrates the sacrament which is the centre of Catholic worship, the Holy Eucharist. That sacrament was instituted by Jesus at his "Last Supper" with his disciples on the night before his crucifixion.
The Eucharist was celebrated from the earliest days of the Church, but in the thirteenth century there was a movement in the Catholic Church of the West for the establishment of a particular feast day in honour of the Eucharist.
Pope Urban IV ordered the observance of such a feast in 1264, and in the following century it became universal in the Western Church. For six hundred years that feast was observed on the Thursday after Trinity Sunday, but in 1970 the new Roman Missal, while retaining that day for some countries, provided that in others the feast be observed on the Sunday after Trinity. The latter is the case in Canada.