Anyone visiting France in September might be interested to know that the so-called Journées du Patrimoine, or Heritage Days (the third weekend in September), offer visits to thousands of historic monuments across France, as well as gardens, museums, theatres...even the Paris sewers...that are either not open to the public or which offer limited access or entry fees.
The concept was introduced by the Ministry of Culture in 1984 as La Journée Portes Ouverts, and today represent a chance for you to access some otherwise inaccessible architectural and cultural heritage. The website tells all, but only in French.
Most sites are free, although a few make a small charge, and some can be visited only as part of a guided tour. You can get details from the website, and if your interest is in French culture and architecture, then it is certainly worth the time and effort to translate the website information – Google Translate will make a passable, if imperfect, stab at translating for you, enough anyway for you to get the gist.
There is plenty of info with short descriptions of locations across Paris and the surroundings areas, but not elsewhere. So, if you intend travelling more widely across France on these two days, it is worth bobbing in to the local tourist information centre and asking which properties, etc., might be open specifically for Les Journées du Patrimoine. If you are staying in Paris, you can get a printed programme from the Ministère de la Culture et de la Communication de Paris (182 rue St-Honoré, 1st arrondisement), open 14-20 September 9.30am-7pm (4pm on Sunday).