Open Monumentendag in Netherlands
Open Monumentendag in Netherlands is held on September 12. Second weekend of September. This event in the second decade of the month September is annual. 
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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.
Similar holidays and events, festivals and interesting facts
World Redhead Day on September 7
World Headache Day on September 12 (A Day of Awareness and Support);
Prinsjesdag in Netherlands or Budget Day on September 15 (celebrated on the third Tuesday in September);
Festival in Leiden, Netherlands on October 3 (Leidens Ontzet - tradition since 1886 in honor of the end of the siege in 1574);
International Day for the Prevention of Money Laundering on October 29
Opening of carnival on November 11 ("Karneval"/"Fasching", on 11-11, at 11:11. Germany, the Netherlands, and other countries);
Sint Maarten's Day on November 11 (St. Martin's Day: Sint Maarten, Kingdom of the Netherlands)