September means the start of Fall, and Fall symbolizes death or change. Throughout the poem, it is talking about change and death as seen when walking through this field. It then moves to a different place (the hospital, the water, etc.) where we are still talking about death and mourning. But then there is a change, and just as Spring comes, which symbolizes birth and growth, there are the frogs, giving the narrator hope among all of the death and mourning they had been experiencing.
