As others have mentioned, the nighttime restrictions at DCA are in effect from 2200-0700 Local time. For Delta mainline aircraft, MD88/90s and 737-700s may land with no restrictions. 737-800 and versions of the 757 may land provided they are below certain preset weights. A319/320 may land using a particular flap setting.

For takeoff, MD90s and 737-700s are unrestricted. A319/320s, 757s, and 737-800s are all subject to certain weight limits. MD80s cannot takeoff during nighttime hours.

If an aircraft was scheduled to land prior to the nighttime hours (2200L) but is running late, it may still takeoff from it's departure airport and land at DCA provided the Approach Clearance is received prior to 2230L ( I can tell you that in the old days when we flew the 727 on the Shuttle and we were running late on that last inbound, we would start asking for Approach clearance somewhere over Pennsylvania....they frown on that practice nowadays!)