This is a Solemnity of the Roman Catholic Church, celebrated the Thursday after Trinity Sunday. In many places the Feast is transferred to the Sunday following the Thursday. It is a Holy Day of Obligation in many countries meaning the faithful should attend Mass. It celebrates the institution of the Eucharist at the Last Supper on Holy Thursday. Today it is called the 'Solemnity of the Most Holy Body and Blood of Christ.'
St. Thomas Aquinas was comission to compose the office for this feast. He wrote the "Pange Lingua Gloriosi" and "Tantum Ergo Sacramentum".
For centuries this feast has been accompanied by a procession of the Eucharist in a Monstrance. These procession typically involve the entire Church walking and singing hymns and prayers.