Corpus Christi existed before the city in Tejas. Corpus Christi is the body of Christ, it is the Sacrament. Holy Thursday, the day of the Last Supper, was the initiation of Corpus Christ and the Sacrament.
It's introduction as a feast was initiated by the urging of St. Juliana of Mont Cornillon and Liège, whom impressed Robert de Thorete, then Bishop of Liège. At that time, bishops could initiate feasts within their diocese. Robert did so in 1246, he died soon thereafter, before the first celebration. Jacques Pantaléon was, at that time, Archdeacon of Liège.
The Bull, Transiturus, was issued in 1264 by Pope Urban IV, whom had been previously Jacques Pantaléon; he died soon thereafter. The feast was extended to the whole latin church. Urban had St. Thomas Aquinas compose a new office for the feast. The songs, Lauda Sion, Sacris Solemniis (which includes, Panis angelicus), Pange lingua gloriosi (which inludes, Tantum ergo) are from Thomas for this day. Another communion hymn of his is Adoro Te Devote.
Even before the changes in celebrations, of the last generation, the US moved this celebration to the Sunday next. It is a public holiday in several lands. It is one of those catholic holidays that disturb the convenience of tourists and commerce.
Soon after introduction, outdoor, theophoric processions became common, and spread to many countries. Of course, with the suppression that came with the triumph of the protestant rebellions, and the imposition of state heresy, such public demonstrations in regard to the faith were absent in several lands. But, to-day with societal apathy being the more pronounced, such public, outdoor processions are an opportunity of evangelisation. People may notice, “Oh, the catholics are having some sort of parade, or something.”
Traditionally the feast is on a thursday, but the procession does not have to be. Since it is a movable holiday, it can fall about 13 June, Saint Anthony of Padua and Lisbon. Saint Anthony is a rightly, popular figure. It has so happened that, adjacent parishes have had, both processions on the same day. What does the bewildered stranger, or unbeliever think?
noto bene: last year, I had another essay.