DUNCAN - Smithsonian magazine has designated Saturday, Sept. 21, as the 15th annual Museum Day, which provides participating museums across the United States a chance to open their doors for free to those who download a branded ticket. The Chisholm Trail Heritage Center in Duncan is again one of those participating museums. More than 15,000 museums are expected to participate in this year's event. To visit the Heritage Center for free on Saturday, visit Smithsonian.com/museumday to learn more and download a Museum Day ticket. Visitors are allowed to download one ticket per email address. The ticket provides free general admission for two people on Saturday, Sept. 21. The Heritage Center is open 10 a.m. to 5 p.m.