I checked. A Yellow-Crowned Night Heron is on the official checklist of birds of the District of Columbia (which, amazingly for DC's small size, lists 325 species). So this is not a rarity.
Still, this is not a bird you see in the nation's capitol every day, so I was shocked, pleased and amazed to find one hanging out at Kenilworth Aquatic Gardens, which is a beautiful place to walk and visit if you ever find yourself in Washington at any season.
Not sure what this bird was doing here, especially at the top of a pine tree (lower picture) where I first found her (him?) I don't think it was part of a nesting pair, and looks to me like a young adult bird.
Seeing it was a great way to start the day. I can now add Yellow-Crowned Night Heron to my list of birds seen in DC. And, er, to my list of birds seen *pooping*