Definitely not during August, when Congress is not in session. The most interesting story I know is from my friend who wandered into a Senate building to use the bathroom and stumbled on her state's senator's office. While taking a picture of the door, his publicity person came out and gave them a private tour and passes to a vote later that evening.
You can also just book a tour online through the Capitol Visitor's Center website:
Visiting the Capitol
Welcome to the U.S. Capitol