What is the significance of the change in weather from the sultry heat from the day before to the "autumn flavor" of Gatsby's last day alive?Notice the season of the year.
The last full day of Gatsby's life is one of the last full days of summer, followed by a day clearly indicating the approach of autumn. Symbolically speaking, summer is the season representing the fullness of life; autumn is the season when living things prepare for their approaching death.
On the "broiling, almost the last, certainly the warmest (day) of the summer," Tom and Daisy, Nick, Jordan, and Gatsby attempt to escape the heat of the weather by traveling into the city. However, the heat of emotions, fanned by jealousy and suspicion and unfulfilled dreams, can not be left behind. Tempers flare, accusations are made, and angry groupings depart at different times.
The next morning, as the weather changed, so did the realities of Gatsby's life. As he came to grips with the fact that Daisy was never going to be his, he was also fighting against accepting the death of the dream he had constructed for his life as Jay Gatsby. By that afternoon, that fight was finished.