World War Two vintage Labor Day poster, produced by the Office of War Information.
The Knights were not the Kiwanis, and they weren't pushing for a "let's be nice to the nice" holiday. The labor movement at that time was large, left wing, and militant.
Indeed, Grover Cleveland had the holiday put on September 1, not May 1, which was the logical date and the one that the Knights would probably have expected and feared, but that would have nearly coincided with the anniversary of the recent Haymarket Riots, so that was not done. And May 1 was the Labor Day pushed by Socialist globally, something that most Americans outside of the Labor movement would have been very concerned about adopting as an American holiday. September 1 became the day, all the way back in 1886.
Labor movements were a huge deal at the time, and they were pushing for workers rights in a large, and radical fashion. Some were very outwardly as radical as can be imagined, others less so, but the movements were extremely powerful. Starting about this time, the more "progressive" elements of American politics started to co-opt and adopt the less radical elements of the labor movements demands, however, and a long period of slow cooperation with labor and politics commenced.
By the 1930s, and the Great Depression, things had evolved to the point where Labor was essentially Democratic, although even as late as the 1940s there were certain Labor elements that were fairly openly Communistic in sympathies. During World War One Labor was not fully cooperative with the Democratic administration, but by World War Two it was, having come to the conclusion during the Great Depression that the administration and the Democratic Party was its ally. Indeed, in some ways the poster set for above is completely correct, and American Labor can take credit for at least part, and a fairly signficant part, of the Allied victory in World War Two.
After the war American Labor entered what may be regarded as its golden era really. The American economy survived the war intact, unlike nearly every other industrial economy, and Labor had, by that time, achieved nearly every goal it had striven for in politics. The 40 hour work week, fairly good working conditions, and many significant goals had entered the American norm.
Perhaps that's why the Labor movement has declined, since the 1970s, to a mere shadow of its former self. Only part of the reason, but part. It became very strong and achieved huge successes, but after that it kept on and demanded further concessions for its workers, in an era when those jobs began to go overseas. While some unions remain strong, none of them are what they were in 1970.
Even the holiday isn't what it once was in a lot of places. In a lot of places, it's just the unofficial end of summer, a three day weekend before students really begin to knuckle down for Fall.
And oddly, at least if Facebook is the judge, it's another holiday that's starting to morph into an additional Veteran's Day. A lot of American civil holidays are now secondary Veteran's Days, and Labor Day certainly wasn't meant to be.
It's an interesting example of a couple of trends. One is the rise, massive decline, and then rise in another form, of American Labor. The other is the intense focus on veterans such that at least three American civil holidays and a couple of unofficial civil holidays are focused on them. And finally, it's an interesting example of how so many American civil holidays are set to make for a three day weekend.