Never seen these. I'm gonna say post-war.
The flag looks like the Stars and Stripes, from what little I can make out of the people it looks like there may be a bunch of derby hats, rather than top hats lol, and I'm doubtful its an Independence Day celebration as from the trees, I would say its in the fall the way people are dressed, or just a cool winter day.
An Election day gathering awaiting the results perhaps? Or perhaps an 1878 inauguration as suggested. A probable clue to when its not would be to find out when telegraph lines were in front of the Capitol building as you can see them near the bottom of the photo. All and all the style of photography looks like a CW era camera was, but that doesn't mean it is.
I wouldn't mind knowing for sure when it was taken.