I stay a lot in NOVA and a Thurs-Sat or Sat-Monday reservation is much cheaper than a Thurs-Fri. Plus you get 2 nights elite & megabonus credit!
Also, for mid-week stays, you might be better off using points than cash. There are a few Cat 1-3 hotels -- especially out towards Dulles that give a great $/point value if you have to stay M-Th.