The Notte Bianca (check the link for up-to-date information) or "White Night" in Florence takes place on the evening of April 30th every year. The streets flow with people making rounds between food stalls, exhibitions, art installations, museums, dj sets, and live music and dance performances scattered throughout the center of Florence.
Events for kids usually start as early as 14.00 and street events commence towards 19.00, but the energy of the city escalates after midnight and continues into the early morning. A selection of museums and palazzi have extended hours, and many shops and restaurants stay open until nearly dawn. Basically it is a giant street party, with a relaxed, care-free vibe.
Here are some images from past events: