Time Out says
New Yorkers sure do love museums, but we appreciate them even more when we get to go without paying. That’s why you should plan on visiting one on September 21, when you can access one out of 31 stellar institutions within New York City. If you expand that net a little wider there are over 100 museums opening their doors in New York, plus 30 in Jersey.
All you have to do is register and download a voucher from the Smithsonian museum. With your ticket, you gain access to participating big-name institutions like the American Folk Art Museum, Cooper Hewitt, Smithsonian Design Museum, Intrepid Sea, Air & Space Museum, Mount Vernon Hotel Museum, The Jewish Museum, The Rubin Museum of Art and more during normal business hours. Check out the full list here and go get cultured!