We have an adjective, "dry," which describes the noun "September." The first line of the story expands on this relationship:
Through the bloody September twilight, aftermath of sixty-two rainless days, it had gone like a fire in dry grass—the rumor, the story, whatever it was. (1.1)
Now we have another adjective to add to the mix, "bloody." Blood, we might imagine, is often wet, in contrast to the "dry" in the title, and the lack of rain through the hottest part of the Mississippi summer. "Bloody" supplies a tone of menace that harmonizes it with "dry."
It's not all of September that's being described as "bloody," but rather the "twilight." This makes us think of a sunset, which can be red, or bloody looking. We can't forget the first word, "through." If the rumor has traveled through the twilight, it has reached night. (We later learn that it is a Saturday night, to be exact.) As we note from the burning grass simile, it is as if the rumor is on fire (the grass is not literally burning, though). This is a fancy way of saying that by the time the rumor reaches the hot night, it is out of control. "Aftermath of sixty-two rainless days" lets us know that the sunset, the spread of the rumor, and its out-of-control status are all products of the dry heat and the violent passions that notoriously accompany hot weather.
So why are we going on and on about the first line when we are supposed to be talking about the title? Well, because the title on it's own is just a rumor. It doesn't sound good, but we could still have a happy story on our hands.
The first line expands on the rumor of the title and gives us the bare facts of the case, which is that something bad is about to happen, and the heat and the dust and the violent passions will culminate in some violent and bloody event – it tells us much of what the author means by "Dry September."
Here's a quote from said author that helps us understand the nuances of the title:
I write about the human heart in conflict with itself, its fellows, with its environment. (source)
It's all there. The bloody human heart, the conflicts between people, that burning rumors, the hot weather are notorious for helping fan the flames. In short, the title presents us with a version of the story pared down to its most basic element – dry weather. And for Faulkner, the weather is only important in so far as it's impact on the people experiencing it.