The problem is that it wasn’t just in Tishri that our ancestors dwelt in sukkot, but the whole year – actually the whole of the forty years between Egypt and Eretz Yisra’el.
If we needed to mark the Israelites’ survival in the desert any date would have worked. But the Torah must have wanted to teach us something special when it specified 15 Tishri.
The explanation seems to be connected with the weather.
In the northern hemisphere it is an in-between season, neither warm and sunny summer nor cold and rainy winter. Sitting in the sukkah at this season emphasises our vulnerability. The sukkah is frail and at the mercy of the elements, and so is our human life.
There is never a guarantee that we will be able to have light, success and security all our lives: nor is there a danger that life will always be dark, depressing and disappointing. We constantly hover between the darkness and the light… but God is with us.
No other time of year would teach this lesson so effectively.