The Holocaust museum is adjacent to the National Mall. There are many Holocaust memorials and museums throughout Europe and the US, but this one in DC is particularly special because of its continued efforts of education toward both memory and genocide prevention.
The memorial museum is one of the most elegant and horrifying efforts at preservation and conservation of this atrocity that has been made. There are other spaces of mourning, like the camps themselves; this made space is a blend of historic memory and emotional truth which seeks to keep that idea, "never again", at the intellectual forefront.
Don't skip the Simon-Skjodt Center for the Prevention of Genocide. Also, in the memorial, brace yourself for the images on screens.
Mon-Su: 10:00-17:30