This is a festival celebrated by the Catholic church and some ‘high’ Anglican churches.
This festival is celebrated on the Thursday after Trinity Sunday and this year (2019) is the 20th.
It recalls the activity of Jesus when he started the celebration of Holy Communion, (or the Eucharist), the origins of this began with the evening communal meal he shared with his disciples (followers) just before his arrest described in all of the four Gospels and which we remember as Maundy Thursday.
In the Church of England the celebration is also known as ‘The Day of Thanksgiving for the Institution of the Holy Communion’ better to stick to Corpus Christi, and has the status of a festival (hence the special service in Church on this day)
Christians believe It is during the eucharistic prayer and the laying on of the priest/vicars hands on the bread and wine that a change happens, christians call it ‘The Transubstantiation’ and I quote, the conversion of the substance of the Eucharistic elements into the body and blood of Christ at consecration, only the appearances of the bread and wine still remain
The Service is very much focused on the Holy Eucharist, highlighting the joy of the Eucharist being the body and blood of Christ, but the service format usually mirrors that of the Sunday services, with hymns, readings, a psalm and the Gospel all being read or sung, and holy communion being taken, however the one difference comes at the end of the service.
There is a parade around the church and in some, outdoors, and at the front of the parade, carried by the priest/vicar is the Monstrance or also known as a Ostensorium, a convenient way to exhibit objects of piety.
This is our one at St Francis.
As you can see there is a window and door in the middle and at this service a consecrated wafer is put into the window and the whole vessel is carried around the church, this is seen to be a blessing by Christ coming to the whole, rather than a blessing from the priest/vicar to the individual.
Sometimes they are used as a reliquary (container) for the public display to pilgrims of christian relics or artefacts.