I'm on board with this, however I view it as a service issue and not really a fare issue. If you're going to run weekend service, then charge weekend rates. Full stop.

But you would have to do something about Fourth of July. I point this out because think about the crowds coming and going to the National Mall and how you have to pay for staff, operators and MTPD to handle that demand. I guess they could try splitting it up so the morning was weekend rates and afternoon was full fare, but the simplest/easiest thing to do would be normal fares for that day.