The National Mall in Washington, DC, is an open-area park right at the heart of city. This area is among the most popular places to visit in Washington, DC, particularly because there are many monuments located here and its proximity to landmarks like the White House, the US Capitol, and the Smithsonian Institute Museums. During the weekends (and summer evenings) this area is also popular among locals who go there to play sports, or simply go for a jog or a walk. On the west end of the National Mall you find the Lincoln Memorial. On the east end you find the US Capitol, and roughly in the middle you find the Washington Monument. It’s about 1.9 miles to go from one end to the other.
I live in the suburb of Washington, DC, so I don’t go to the downtown area on daily basis. But often times I would go there during the weekends especially either to visit one of the museums or if there is a special event (festivals, charity walks). The National Mall is pretty nice (and safe) to visit at night as well. You can see the Washington Monument and some of the memorials lighted up. During special days like Memorial Day or Fourth of July (or presidential inauguration), the whole area could be packed with people coming to attend concerts, fireworks, etc.
The photo below was taken from the steps of Lincoln Memorial. You can see the Reflection Pool, the Washington Monument, and the US Capitol in the distance.