It's in September to keep it from being on 1 May, where it is in the rest of the world and where it has a deep and abiding connection to Socialist and Communist politics. The American labor movement wasn't without such connections, but it played out differently here, and so, hey, we get a nice end-of-summer holiday.
You can read a lot into it -- you can go wave a purple or red sign and march with whoever, if you like, this being a free country and all -- but the reality is, that's all it is now, a nice holiday marking the end of summer. Enjoy it.
10 months ago