Corpus Domini or Corpus Christi is a festival in honor of the Eucharist, the "body" of Christ symbolized with bread. It has been celebrated 60 days after Easter Sunday on the Thursday after Trinity Sunday since the XIII century.
This occasion is celebrated all through Italy and in Florence where, during a procession, the Eucharistic bread is carried through the streets of the city in a crystal ostensorium (a receptacle in which the consecrated Host is exposed for adoration).
Between 1311 and 1312 the ecumenical council disposed the rule that during the ceremony the relics of the saints were to also be carried in the procession.
Celebrations were originally held inside the church of Santa Maria Novella but once the church of Santa Maria del Fiore was built, the Canon decided that the celebration should be moved there making it the starting point of the procession as well.
This decision brought disputes between the Dominicans of Santa Maria Novella, where the ceremony took place, and the clergy of the new Santa Maria del Fiore. The disputes were concluded in 1458 with a bill passed by Pope Pio II which established that the procession be moved to the newer church of Santa Maria del Fiore.
The celebration was enriched with more splendors by the Grand Dukes of the Medici's who, in the 1500's summoned the town's citizens, who lived along the streets of the procession, to adorn their houses with flowers, festoons and tapestries.
Today the procession starts at the Duomo, but has been shortened due to traffic and other modern day obstacles, and the route and the final destination church is chosen year by year.