It is the Feast of the Epiphany (Festa dell' Epifania), which celebrates the manifestation of Christ to the Gentiles as represented by the Magi (Matthew 2: 1-12) And Florence will celebrate the day in style with the Cavalcata dei Magi.
The Cavalcata dei Magi, a grand cavalcade with people in historical costume, is a fairly recent (1997) revival of a tradition which goes back to the 15th century when it was organised by a group known as the Compagnia dei Santi Re Magi also known as 'La Stella'.
To celebrate the Feast of the Epiphany, 'La Stella' organised lavish pageants and processions, which involved hundreds of people on horseback and many more on foot. The Medici family were active members of La Stella.
Nowadays, the cavalcata is held every year. However in the 15th century it was first held every three years and, after 1447, every five years. It was suppressed in 1494 with the exile in that year of the Medici family.
In the 15th century the Cavalcata dei Magi was made up of three separate processions, which met up at the Baptistery before making their way, en masse, to the church of San Marco.
The modern version is a single procession, which starts at the Piazza dei Pitti and ends at the Duomo. And in the 21st century, unlike in the 15th, few of the participants, apart from the three Magi, are on horseback.