These heritage days were born in France and are now europeans. They occur on the 3rd week end of september (19-20th in 2015). For 2 days, most of the monuments and museums in Nice are open and free. Some places, such as the former Sarde palace or the workshop of the Opera sets which can be visited usually welcome visitors.
In 2017they will take place on september the 16th and 17th.