Florence is at its busiest in spring (April – May) and autumn (Sept - Oct), when it's warm but not hot, and you can coincide with some festivals: a 'cart explosion' at Easter, musical events in May, a torchlit procession on September 7th, wine harvest festivals in October. June is also popular, and on the 24th you can witness a 'medieval football' match (often closer to rugby or wrestling) in the Piazza Santa Croce. In midsummer the city can get pretty stifling, so most Florentines head up to the hills, and most tourists follow suit. Despite the reduced rates in some hotels, avoid August, when some attractions and restaurants close. Winter (Nov – March) is a genuinely attractive time to visit, with plenty of local activity, but much more space in the galleries and museums. Hotels do get booked during fashion shows, the biggest of which are the Pitti Fairs in January and June, and the Modaprima Womenswear in late November. Christmas-New Year is a great time to come, with festive lights and fireworks.
See Tuscany for details.