On September 29th, 2019, the Monuments' Day 2019 takes place in all 9 Austrian states, most notably in Vienna. This event is Austria's contribution to the European Heritage Days, organized by the Federal Monuments Authority of Austria, which oversees all protected buildings and monuments.
On that day hundreds of buildings, all being subject of a preservation order, can be visited for free, some offering guided tours, some normally not open to the public.
Only in German: https://tagdesdenkmals.at/
Click on "Programm" and select the state for details, e.g. "Wien" for Vienna.