Saturday September 26th is Smithsonian Magazine Museums Day

This is an event where participating museums allow two guests, with a downloaded visitor pass, to visit at no charge.

-- Smithsonian Magazine Museum Day website

Unlike "Doors Open" events in other countries and cities, only "one door" is fully open, not dozens or hundreds.

-- European Heritage Days is a coordinated program of making cultural resources open and free to the public in a concerted way across the countries and districts of Europe.  Doors Open Days Scotland represents that country's participation in EHD.

-- Heritage Open Days UK.

However, there is no question that it is easier to do "Doors Open" at multiple sites in one city, and it maximizes impact, which is difficult to do on a countrywide basis.

-- Doors Open 2015 - City of Toronto is in the Spring
-- Doors Open Milwaukee, was last weekend
-- Open House Chicago, October 17th-18th