Best time to visit Florence is from May till September, that offers a nice summery weather and many open-air events. During this time, the city is highly crowded, and hotel rates are at their maximum. Reservations must be made three to four months in advance.
Early spring, between March and April has mild weather and better accommodation deals. Winter, and especially the month of February is also another good choice, if you do not mind chilly weather, as it is quiet and inexpensive.