I just learned that Sept 30th is.
Free entry available to museums around the country on Saturday, 30 September 2006, as part of the Smithsonian-sponsored Museum Day. You have to download the Museum Day Card to gain free entry for 2 people. Visit the site for the card and the list of participating museums (by state). For those in the DC area who are used to free museums, here is your chance to visit some of the museums in Baltimore without paying a fee (including the Maryland Science Center).
How neat is that? When I was in Florence on my honeymoon they were offering free entry to all of the museums – but it was a weeklong (or monthlong event). Anyway, make sure that you take advantage of this great offer!