April is a very busy month in Florence, with the biggest event of course being Easter - or Pasqua - as it is called in Italy.
One of the most wondrous of events is the "scoppio del carro", or in English, the "explosion of the cart" (of St. John the Baptist). The cart itself is also known as "Il Brindellone". This medieval Florentine tradition is a spectacle not to be missed. The ceremony started sometime during the crusades, and the original cart was said to be lit by a flint brought back from Jerusalem by members of the Pazzi family. The cart gets led into Piazza Duomo by two white oxen amid much fanfare, flag throwing, and drum playing. It is covered in various fireworks and other exploding devices, and lit by a fake dove that comes shooting out of the Duomo doors on a wire. The mass starts at 11 AM, the cart usually gets lit around noon. Do not miss it! (Not an event for the noise adverse.)
That is just Easter. Other major annual events are the Firenze Half Marathon (April 6th), the Mostra Internazionale dell'Artigianato (April 24th to May 1), and of course, Notte Bianca (or White Night) - the annual all night street party (one of my favorite events of the year).
There are also many other small markets and fairs in April, see our Florence Events Calendar for additional dates.