The Smithsonian Museum of American History in Washington, D.C., will be closing on September 5 for major renovations, and won't be reopening until the summer of 2008. Labor Day will be the last day to visit the museum. So if you've been meaning to go, this summer is your last chance for two years. On the flipside, procrastinators who don't get there until after it reopens will be rewarded (eventually) with an architecturally improved, much updated museum. It's business as usual until the closing date, with free admission and extended summer hours.
Some exhibitions have already closed, but many of them will remain open until the September closing date. The museum's website lists accessibility by exhibition. So, for instance, you still have time to see Julia Child's kitchen and the John Bull steam locomotive, but the national firearms study gallery and the Ella Fitzgerald exhibition are already closed.