The arrival of the Carro Matto to Florence occurs on the last Saturday in September. It is a part of the celebrations of the "Bacco Artigiano" that take place to celebrate the end of the grape harvest in the Chianti Rufina region.
It used to be on September 29 to celebrate St. Michael's but now just takes place on the last Saturday of the month, hopefully you'll be here around that time since it is an incredible display!
For anyone that will be in Florence this weekend - the Carro Matto and festivities will take place this Saturday starting from around 3.30pm. There is a parade that starts around that time and goes through the historical center. Arrival in Piazza della Signoria is around 5pm.
The event ties in with celebrations taking place in nearby Rufina in honor of wine, local artisans and music. If you head out toward Rufina on Sunday several wine estates will be open for tastings and visits. There is more information on http://www.baccoartigiano.it/ but only in Italian. I'll post some more information on our blog later on.