The festival of Corpus Christi celebrates the Eucharist as the body of Christ. The name 'Corpus Christi' is Latin for 'the body of Christ'.
This jubilant festival is celebrated by Roman Catholics and other Christians to proclaim the truth of the transubstantiation of bread and wine into the actual body of Christ during Mass.
In some countries in the world, Catholic churches still celebrate the festival, not only with a Mass, but also with a procession that carries the consecrated wafer through the streets as a public statement that the sacrifice of Christ was for the salvation of the whole world.
Corpus Christi falls between late May and the middle of June, on the first Thursday after Trinity Sunday (60 days after Easter). In some countries the festival is celebrated on the Sunday after Trinity Sunday.
In the Church of England this feast is also kept on the Thursday after Trinity Sunday and known as the Day of Thanksgiving for the Institution of Holy Communion (Corpus Christi).
It's worth noting that Christians already mark the Last Supper, when Christ instituted the Eucharist, on Maundy Thursday (the day before Good Friday). Because Maundy Thursday falls during the solemn period of Holy Week, it was thought necessary to have a separate festival of the Eucharist that would allow the celebration not to be muted by sadness.