Florianopolis lies not only below equator but also below the Tropic of Capricorn, which among other things means that summer and winter seasons are very different. Summer in Brazil runs from December from February and weatherwise this is the best time to visit Florianopolis. In the winter (June, July, August) lowest temperatures often oscillate between 8 and 12 Celsius (46 to 54 F).
Most people in Brazil and neighbouring countries have summer holidays in January and so Florianopolis can get little crowded. This might be the best time to party, but you might pay premium for accommodation, transportation etc. If you want a nice warm weather then you should come early in December or in February (definitely after the Carnival)