This is a casual affair. The end of summer can still be hot in VA. Ladies might feel comfortable in a casual dress or shorts/pants with a nice shirt and sandals; men in a light weight shirt with nice shorts or slacks. I would not recommend heels since the might sink into the grass.
Since this is at our house, parking is limited. Please uber.
Our house is 2 miles south of Old Town Alexandria, which has tons of restaurants and nightlife that are walkable. This is a great area to stay.