I doubt that there was any pre-announcement notice to draw crowds, but a OFA blast notice would have gone out to thousands and thousands of college dopes. Look at the coverage, nearly 100% white, non-anarchist looking college kids made up the crowds in both cities. That tells me that it was not spontaneous, that they all got the memo.
“Real big announcement soon. Come to the White House/central gathering place and bring markers and posters... Details to be announced...” SOP
I figure they would not give the details, just round up the usual lemmings who know the drill, either Political Party I might add play this game.
Ignorance is bliss, the cynics always miss out on all “the fun”.