A theme of growth is also possible. Just as the plants start to change and prepare for the colder months, characters in the story could be growing in different ways. It could be emotional growth, like learning to be more independent, or physical growth in a more metaphorical sense, such as becoming more resilient in the face of upcoming challenges.
I'm not sure which specific 'ahh september short story' you mean. There could be many short stories related to September. One could be about a person's new beginnings in September, like starting school or a new job. It might describe the changing of the seasons, the crisp air, and the sense of anticipation that often comes with this time of year.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.