The unmentionable odour of death Offends the September night.
Auden is speaking here about the way in which we ignore things that we sometimes do not want to address. He calls out people, almost in a way that suggests their private lives are trivial, compared to the public events affecting all people. He speaks of the "unmentionable odor of death" as something that is ignored, yet should not be, as if it is the elephant in the room. A topic that is almost too big to comment on, death, so the decision to not speak about it seems easier. The way we cannot speak of the death happening in the world is mocked, as conversation of this would ruin the September night. Thus, showing that there is no time and place to discuss big, hard topics like this, and if you wait for that time it will not come.