Open Monumentendag, the Netherlands
The Open Monumentendag (Heritage Day) is designed to bring people into contact with the historic environment and to encourage interest in and understanding of historical monuments and the need for their preservation. During Open Monumentendag thousands of historical buildings and sites (about 4.000) are open to the public free of charge. Besides opening their doors, many locations also organize on-site activities like exhibitions, musical performances and guided tours. Each year around 80 to 85 percent of Dutch municipalities participate in the Open Monumentendag, organized by local committees.
Open Monumentendag is part of European Heritage Days, a joint action of the Council of Europe and the European Commission. Every September, millions of people visit heritage treasures across the 50 States party to the European Cultural Convention.
You can find more information about this event here