Take a look at the websites for the American Art Museum/National Portrait Gallery http://www.npg.si.edu/exhibit/current.html and http://americanart.si.edu/exhibitions/ - two of the very best museums in DC for photography, and they're in the same building. The National Gallery of Art has a photo department but it's still a bit of the redheaded orphan bastard stepchild of the collection so it doesn't get much love (I'll make an exception for the current Harry Callahan show). FotoWeek DC will be on that weekend, with lots of really cool things happening. Sunday the 6th, there will be an all-day alternative process love-fest at Photoworks Glen Echo (at which I'm one of the presenters, showing platinum prints) but that would be a bit of a challenge to get to if you're not driving, since the streetcar line that went to their front door was taken out in the 1960s. You'd have to take Metro to either Foggy Bottom or to Bethesda and then get at least one bus.