There’s only one problem. Since 2010, September has averaged a slight gain, not a big loss.
Rules of thumb were helpful guideposts for many years but now seem to be less helpful, especially since the global financial crisis of 2008/9. Regular government-released economic reports also seem to have more measurement questions than before the crisis. There is no conspiracy about all this, but there has been a fundamental change that we are not measuring correctly. I cannot articulate what that change has been, but I’m glad some serious thinkers are working on it.
The good news is that the last three months of the year are historically very good months in the stock market. I suspect it may be a little less good this election year, as the market has already pulled the post-election gains forward to the Spring/Summer, thinking it knows that Clinton will win.