Which is the best route to take from Nashville to Washington, D.C.?

In June, 2012, my son is graduating with his Masters Degree. He has a 4.0 average and is graduating with honors. I would not miss this for anything. Since I have never been north, beyond Indiana, which Interstates should I take and what is the traffic like in Washington, D.C.? And, are the people as cordial in Washington as those of the south? I have a GPS that I am going to rely on, but my GPS does not tell me what to expect from the people. And, is June a good time to go to Washington? Question: i am relying on my fellow Fluterers for some good information, so can you be of help?