Due to the fact that the contract employees were stealing money the collection was automated at metro garages a year or two ago. After 10 pm (or whenever) on Friday it's open season to park over the weekend. They may have staff looking for multiple day parking on weekdays. But you could probably get away with parking on Friday anytime and getting out before Monday am.
I even confirmed that the it is technically legal to park > 24 hours by calling WMATA. During it's max 24 hours -- not sure how it is enforced in practice.
I thought the public garage meters in Bethesda are in operation on Saturdays but maybe I am wrong. They have lots of info on the Montgomery Cty website. If you are driving from Pittsburgh then that's a good place to park.