Like Passover and Shavuot, Sukkot has both agricultural and historical components.
Historically, Sukkot commemorates the 40 years which the Children of Israel wandered in the desert after leaving the slavery of Egypt before entering the Land of Israel.
"...that your generations may know that I made the people of Israel dwell in sukkot (booths) when I brought them out of the land of Egypt" (Leviticus 23:43)
Sukkot marks the end of Israel's annual agricultural cycle, and is also referred to a Chag ha-Asif (Festival of Ingathering)
"You shall keep the festival of ingathering at the end of the year, when you gather in from the field the fruit of your labor." (Exodus 23:16)
"You shall keep the feast of sukkot (booths) seven days, when you gather in from your threshing floor and your wine press". (Deuteronomy 16:13)
With the agricultural year ending on Sukkot, the early Israelite farmers could rejoice without trepidation: the barley and wheat had been harvested and brought in from the threshing floor; figs and raisins had been taken in before the first rains from the rooftops where they were drying in the sun; the olive harvest had begun, and date clusters were ripening on the date palms.
A central aspect of the celebration of Sukkot is the "four species":
"And you shall take on the first day the fruit of a goodly tree, date palm fronds, the branch of a leafy tree, and willows of the brook; and you shall rejoice before the Lord your God seven days" (Leviticus 23:40)
These four species represent the early history of the nation of Israel:
Date palm fronds grow in desert oases and signify the 40 years that the Children of Israel wandered in the Sinai wilderness.
Willows of the brook grow along the banks of the Jordan River where the Israelites encamped before entering the Promised Land
The branch of a leafy tree comes from a tree with dense foliage that the Israelites found upon entering the Land of Israel, which needed to be cleared for the nation-building transition from nomadic shepherding to settled agriculture. The Talmudic Sages determined that this "leafy tree" was the myrtle, which still grows abundantly in uncultivated mountainous areas of Israel.
Fruit of a goodly tree is, like the "leafy tree," not identified in the Torah. The Talmud teaches that this was the etrog (citron), considered a "luxury" crop because it is very difficult to grow in Israel. It is the only fruit among the Four Species and it represents the zenith of Israelite agricultural development.
For more information on the history and agricultural connections of Sukkot, please see the book "Nature in Our Biblical Heritage" by the founder of Neot Kedumim, Nogah Hareuveni.