I thought this bit of information from Jennifer Hill, Patuxent Research Refuge, might be of interest to birders. On selected Tuesday evening, the North Tract and the South Tract (where the National Wildlife Visitor Center is located) grounds will be open until 7:30 p.m. Although it isn't dark at 7:30 p.m., at least it is an opportunity see see what birds might be around at that time at those locations.

Twilight Tuesdays: evenings where the grounds will be open until 7:30pm for the general public. 
North Tract - June 2, July 7, August 4
Visitor Center - June 9, July 14, August 11

Marcy Stutzman
Russett, MD