Museum Day is a nationwide event taking place on Saturday, September 29, 2007 where participating museums and cultural institutions across the country offer free admission to Smithsonian readers and Smithsonian.com visitors, allowing for one day only.
The article is here http://www.smithsonianmag.com/museumday/
Check your state for locations here http://www.smithsonianmag.com/museumday/articles/venues.php