I highly recommend visiting Washington DC on Memorial day or July 4th. We have been there on for both. Memorial Day weekend has rolling thunder which is a parade of thousands of morotcycles through the streets of DC. I have never ridden a bike but it is amazing to see. Many of the bikes have flags observing fallen soldiers.
The 4th of July has activities on the mall and fireworks over the Potomac as the grand finale. This is well worth the visit. One word of warning, 2 million people going to the metro at the same time leads to very overpacked subway cars and long waits to leave. We stayed in Alexandria and if I had known, would have stayed inside DC or Georgetown where I could walk after the show.