France invented the format in 1984 and the Council of Europe adopted it. On one September weekend around 17,000 buildings open free, including a category that is otherwise entirely closed: the Élysée Palace, the Hôtel de Matignon, the ministries, the Senate, the Conseil d'État, military installations and the private hôtels particuliers of the Marais.
The ministry's most widely cited figure is around twelve million visitors, from 2011. It is an estimate aggregated from site returns and it is old; we show the year rather than let the number pass as current.
The practical shape of the weekend is queueing. The Élysée has produced lines of several hours, and most of the high-demand sites have moved to timed online booking that opens a few weeks ahead and is gone in minutes.
The pleasure is elsewhere: the working buildings — a water treatment plant, a theatre's workshops, a town hall's attic — which are open to anyone who turns up.
PlanningAnchored to the third weekend of September; the headline buildings issue timed tickets weeks ahead.