Here is the official site of the 2016 Gelato Festival - the final event of the season is September 1st to 4th, at Piazzale Michelangelo:
THE FIRST TWO WINNERS FROM EACH STAGE OF THE TOUR WILL CHALLENGE ONE ANOTHER IN THE GRAND FINAL IN FLORENCE.
18 OF EUROPE'S MASTER GELATO MAKERS
WILL ENTER THEIR OWN UNIQUE FLAVOURS IN AN EFFORT TO BECOME
THE OUTRIGHT WINNER OF THE 2016 GELATO FESTIVAL
PURCHASE YOUR GELATO CARD IN ADVANCE AND COME ALONG AND CHOOSE THE FLAVOUR OF THE YEAR.
One of Florence's newer annual events, the Gelato Festival has been happening for several years now, usually around the end of May (in 2015 the event will also come back to close the tour in October in Florence).
These comments were about the first festival in 2010:
"We had lots of great gelato, but the lines were long at times and the overall organization seemed a bit lacking. It did seem to be a big success however judging by the number of people eating ice cream. The "Gelato University" put on in Piazza Santissima Annunziata by industry giant Carpigiani was also packed for every lesson."
This year's event (2012) is scheduled for May 23rd to 27th and will take place in the following piazzas:
I think last year's edition was better than the first, and maybe 2012 will be their best yet. There is much more going on than just tasting gelato of course - visit the official website for more details (in Italian only).
These photos are from 2010 and 2011: