The 13th edition of culture week (XIII Settimana della Cultura) will kick off in Italy from April 9th to the 17th.  During this time all Italian state museums and historical sites are open for free, they will waive their entrance fees.

This includes the following major attractions:

Florence - the Uffizi galleries, Bargello, Accademia, Pitti Palace
Rome – Colosseum tickets and Colosseum tours, Borghese Gallery, Rome Cards
Milan – Brera Gallery
Venice – Venice Academy

For many, you will still have to book a reservation, but the actual ticket price is free (gratis).  The event does not encompass church owned museums and property like the Vatican museum and the catacombs (Rome, Palermo).  Also of noteworthy exclusion is the Palazzo Ducale in Venice.  Guided tours and other excursions are also not covered.