ella, the festival you are asking about is called scoppio del carro (explosion of the cart) and is held every easter sunday in the piazza del duomo. here's a link to a page which will give you a bit of history, but if you put "scoppio del caro" or "explosion of the cart" into google, a ton of results will pop up. having learned about it in an italian class i took, i have always wanted to see it but never have been in florence for easter. i hope you enjoy it.