Why is Sukkot celebrated in the autumn?

The sukkah commemorates the Clouds of Glory that protected our ancestors in the Sinai Desert, clouds that accompanied them starting when they left Egypt on Passover, during the spring-time. Nevertheless, the Torah specifically commands us to sit in the sukkah during the "seventh month," at the onset of fall.

Several reasons are given for the timing of this holiday:

1. After filling the storehouses with all the produce that was harvested at the end of the summer season, a person might feel confident about his financial situation, and forget the Creator who supplied him with all this material wealth. We therefore sit in the sukkah and contemplate our sojourn through the desert, a time when we had nothing – no fields, orchards, or vineyards – and G‑d alone provided for our every need.
2. If we would sit in booths in the springtime – the season when we actually left Egypt – people would erroneously conclude that we are moving outdoors to enjoy the pleasant weather. We therefore sit in the sukkah during a cool, rainy season, so that it is patently obvious that we are doing so only at G‑d's behest.
3. According to a differing understanding of the verse commanding us to sit in the sukkah, the booths commemorate the actual huts that the Jews constructed and lived in while sojourning in the desert. As mentioned before, the Jews left Egypt at the commencement of the spring season. For the next few months, the weather was pleasant and did not necessitate the building of shelters. Only with the approach of autumn, and with an eye towards the winter, did the Jewish people erect their "sukkahs."
4. According to the teachings of Chassidut, the holiday of Sukkot is directly connected to Yom Kippur, which precedes it by five days. Click here for more on this topic.