15th Tishri is the day when Sukkot celebration begins. Sukkot is the final of the Shalosh R'galim (three asupicious pilgrimage). Sukkot carries a dual importance: agricultural and historical.
The word "Sukkot" stands for "booths," and points to the superficial dwellings that Jews are asked to live in during the holiday in the memory of the period of loitering.
Sukkot festival runs for seven long days. No work is allowed on the first and next day of the celebration. For rest of the days, work is permitted. The days on which work is allowed are called Chol Ha-Mo'ed, or the days of Passover.
To salute historical significance of the holiday, people are instructed to dwell in temporary huts, just like their ancestors did in the desert. The temporary shelter is termed as a Sukkah.
The Sukkot activities and events are big fun for the children. Constructing the Sukkah each year caters to the common childhood fantasy of building a castle, and dwelling in the Sukkah. It satisfies a child's urge to camp out in the backyard. The instruction to "dwell" in a Sukkah can be satisfied just by eating all of one's meals under the hut; however, if the weather and one's health allows, one should spend more and more time in the Sukkah.
In the construction of Sukkah, two and a half walls is mandatory which should be made of the articles that must not damage in the wind. The "walls" of the Sukkah must not be cemented or solid; canvas nailed down or tied is permitted and quite common in the US. The roof should be made up of articles called Sekhakh (covering).
You can also purchase do-it-yourself Sukkah from several online sources available out there, or you can create your own.
It is quite a general practice to decorate the Sukkah. In the US, Jews generally decorate dried corn and squash in the Sukkah to embellish it, as these items are easily available at that time of Halloween and Thanksgiving celebrations. A number of families hang artifacts drawn by the children on the walls.
Another observance during Sukkot activities and events are incorporation of Four Species (arba minim in Hebrew) or the lulav and etrog. People are commanded to add these four plants in the festival and facilitate them to "rejoice before the Lord." These four species are an etrog (a citrus fruit identical to lemon; in English it is termed as citron), a palm branch (lulav in Hebrew), two offshoots of willow (aravot) and three myrtle branches (hadassim). The six branches are tied together and called collectively as the lulav, as the palm branch is the largest branch. The etrog is separated with other branches. With these four species in hand, one chants a blessing and waves the species in all directions (up and down, east, south, west, and north) manifesting the truth that God is omnipresent.
The four species are waved during the Hallel recitation in community services, and are also held during congregation around the bimah (the pedestal where the Torah is recited). These processions celebrate identical processions around the pedestal of the earlier Temple in Jerusalem. This segment of the service is called as Hoshanot, as during the procession, people chant a prayer, "Hosha na!" (Please save us!). On the last day of Sukkot, seven circles are taken. Because of this reason, the last day of Sukkot activities and events is termed as Hoshana Rabbah (the great Hoshana).
After the circles on Hoshana Rabbah, people beat the willow offshoots five times against the floor shaking loose some or almost all of the remaining leaves. A number of justifications are manifested for this practice, but the basic reason appears to be agricultural: in Israel the rainy season begins in the fall, and the leaves falling from the willow branch depicts the desire for much awaited rainfall.
Why are specifically these four plants are used and not any other plants? The two general explanations of the symbolic justification of these plants are - they depict several parts of the body, or that they represent different types of Jews.