September in Florence is a funny month. All of the locals come back from their beach based holidays and ease back into everyday life.
There is some cool stuff happening in the city including "Wine Town" which includes various stalls being set up around the city where you can try many different varieties of wine. Wine Town normally happens in late September.
Also happening on the 7th is the "Festa della Rificolona". A culture rich festival where locals of all ages get involved in a lantern parade lining up along the Arno river. There is also a large party in the "Piazza Santissima Annunziata" featuring street performers, live music and local food vendors.