On Saturday night (19th May) Italian state museums and archaeological sites will be open free from 8pm to 2am, with special events taking place. Highlights include night-time visits to the Villa d'Este at Tivoli, readings from the classics in the archaeological museum in Venice, and music at the Uffizi gallery in Florence.
You see a list of participating museums and search for events by region at http://www.beniculturali.it/mibac/export/MiBAC/sito-MiBAC/Contenuti/MibacUnif/Eventi/visualizza_asset.html_305741442.html
It's a good way to save money and to see sights in a completely different way.
Also taking place this weekend is the Sensa festival in Venice (see What's on in Venice).