Every year, sixty days after Easter, the Church celebrates "Corpus Domini": a religious solemnity in honor of the Eucharist (the 'body' - corpus - of Christ in the sacramental sign of bread): an observance that first developed in Italy the thirteenth century and in 1263 was extended by Pope Urban IV to all of Christian Europe. In Florence as elsewhere, from that period the feast has been celebrated in solemn fashion, with a majestic procession in which the Eucharistic bread is borne through the city streets in a glass container know as a 'monstrance', which allows people to see the consecrated bread wafer. This procession acquired ever greater importance with the passage of time.
Originally the celebration was limited to the interior of the Dominican church of Santa Maria Novella. It took place after Vespers, with the participation of the Bishop, the Cathedral clergy, the City authorities, the populace and representatives of the lay confraternities.
After the Council of Vienne (1311-1312), held during the pontificate of Pope Clement V, the City Government decided to make the celebration public, increasing the solemnity of Corpus Domini by bearing in procession with the Eucharist relics of the saints.
The church of Santa Maria Novella was then the biggest in Florence, the new Cathedral of Santa Maria del Fiore being still under construction on the site of the old duomo of Santa Reparata.
Even so, the number of people taking part became so great that the Dominican church could not contain them, and the celebration was held outside, in the city streets.

Circle of Bernardo Daddi, Our Lady of Mercy. Detail, Cathedral Square with the new duomo under construction. Loggia del Bigallo.

When the new Cathedral was nearly complete, the Chapter of Canons began celebrating the rites there, and insisted that the Eucharistic procession begin from the Cathedral. There arose a lengthy dispute between the Dominicans of Santa Maria Novella, supported by the Dominican Archbishop Saint Antoninus (+1459), and the Cathedral clergy. The dispute ended in 1458 with a papal bull of Pius II, who established that the procession should indeed start from Santa Maria del Fiore. Emerging from the Cathedral, the cortège passed through the Baptistery, went to Piazza della Signoria and thence to Santa Maria Novella, where one of the Cathedral Canons said Mass, returning finally to Santa Maria del Fiore. The tradition of Mass at Santa Maria Novella lasted until 1920.
Fifteenth-century descriptions and illustrations attest to the splendor of this event in the Early Renaissance.

The Corpus Domini procession, in a miniature from one of the liturgical codices of Santa Maria del Fiore (15th century). Fondi edili, Laurentian Library, Florence.

In the sixteenth century, after the fall of the Florentine Republic, the Medici grand dukes took part in the procession in their princely regalia, thus conserving and increasing the magnificence of the occasion. They encouraged citizens to adorn the facades of those houses before which the procession passed with tapestries, ornamental hangings, flowers and garlands, and later forbade the movement of pedestrian and carriage traffic along the processional route.
This solemn festival occupied the whole city. The preparations began three days before the feast itself, when people hung the street awnings that would shelter participants from rain or direct sunlight, and adorned the shop facades with red and yellow bunting.
On the Feast Day, the procession unfolded with great pomp. First came the children, then the lay confraternities, then friars and monks of the different religious orders, then the clergy of San Lorenzo and of the Cathedral. Finally came the Archbishop, carrying the Eucharist in a monstrance under a baldachin (a portable canopy). The rich baldachin in cloth of gold, called "of the Republic", used for centuries on this occasion, was in fact made and paid for by the Florentine Republic. Several days before the Feast, the City Administration entrusted it to the Cathedral Chapter. Unfortunately the tragic flood of November 4, 1966, nearly destroyed it. Since then it has not been used; the City has though restored it and given it in charge to the Opera del Duomo.
All the city's magistracies and the heads of the guilds also took part, accompanied by the soldiery and by musicians. The processional route was strewn with flower petals and laurel leaves, which citizens sold in Piazza del Duomo for the occasion. When the procession ended, the festival continued in the city streets where stands selling sweets and drinks were set up. This solemn festival has never been neglected by the people of Florence, not even for very grave reasons such as inclement weather, wars and sieges.
Today the procession still leaves from the Cathedral, but the route and destination-church are decided year by year. The processional route is shorter, due to the problem of city traffic.