I get where this comes from (look up why USA and Canada celebrate Labor Day rather than International Worker's Day) but it's just symbolism, a frivolous issue. Pushing frivolous issues reinforces one of the negative perceptions of the American left common among both centrist Americans and international leftists. People want healthcare, vacation days, material improvements. Not a debate over when a national holiday is.
Everytime an American on the left does this, they just set back their own cause. It's enough to make you believe a conspiracy theory of some powers on the right actively encouraging it.
You make it sound like the American left pushes a lot of symbolic issues. Which ones are you thinking of?
Hot take: the American left loses on the material issues because it has lost on solidarity, symbolism, ideology. Don’t you know that healthcare and vacation days will “hurt the economy”? There are many poor working class people who still oppose Obamacare even though they directly benefit from it. The material gains matter a lot less if people reject the ideas behind them.
I’m not sure if the timing of Labor Day is important, but I wouldn’t underestimate the power of symbols.