This image is also courtesy of r/historyporn.
Apparently, according to u/DdCno1, this is an image of Reichserntedankfest or like a Thanksgiving Celebration of the Reich.
For comparison sakes, here’s an image of an actual Nazi rally:
I dig black and white photographs, as that gives pictures a better historical sensibility, but there’s something more tangible about historical pictures that have been colorized.
Regardless, the first picture of the festival is undeniably stunning because of the sheer mass of humanity present. Goodness, I love history.