Stay in Sukkos (booths) seven days...So that it will remain known for generations that I housed the Jews in Sukkos after liberating them from Egypt" (Vayikra 23:42-43). Halachah (see Shulchan Aruch O.C. 625:1) defines the booths in which the Jews lived in the Wilderness as the "Annanei HaKavod," or the Clouds of Glory, which were provided by Hashem to shield the Jews from the harsh environment of the desert. In remembrance of Hashem's kindness, Jews were commanded to stay in booths every year so that they are
always reminded of Hashem's miracles during that time. Although the miracle occurred immediately after the Jews left Egypt (Passover time), nevertheless we do not celebrate Sukkos then, for building a shady structure at the onset of summer could be misconstrued as intended for one's personal enjoyment, whereas at the onset of the winter when people tend to stay indoors it is obvious that the Sukka is built for the purpose of fulfilling Hashem's commandment, and thus we are reminded of the Wilderness Sukkos (Mishna Berura there).