On Pesach, at the Seder, we express the idea that "In every generation, each person is obligated to see himself as if he were one of the redeemed Jewish slaves." Similarly, on Sukkot, as we sit and enjoy the festive meals, we are privileged to have as our company seven of the greatest leaders of our People. As Eliyahu HaNavi visits each Seder Table on the Night of Pesach, so on each of the Seven Nights of Sukkot, one of these seven is our main guest, while the others of the "Faithful Shepherds" sit with us as well.
The presence of these great guests reminds us of the supreme importance in the Jewish Religion of having guests, specifically poor guests, or those who are less fortunate than us in some way, to share the joy of the festivals with us. For, as the RAMBAM reminds us, any kind of physical enjoyment which is not shared with the poor or less fortunate, is viewed by G-d as alien, and as only the enjoyment of our bellies. Whereas, if it is shared, that same physical celebration is raised to the heights of "Avodat Hashem," Service of the L-rd.
The seven are our three Avot,: Avraham, Yitzchak and Yaakov, our great teacher, Moshe, and his brother, beloved to all of Israel, Aharon HaKohen, Yosef the Righteous, and the "sweet singer of Israel," who also taught us how and when to fight for the Name of G-d, King David. David was also the ancestor of the "Mashiach," the Messiah, hopefully already born, "May he come soon and in our time!"
Each of these seven endured and overcame the difficulties of exile with the protection of Hashem:
Avraham was commanded "Go forth from your homeland, from your birthplace and from the house of your father, to the Land that I will show you." (Bereshit 12:1)
In connection with Yitzchak, we find "And there was a famine in the Land, in addition to the one that occurred in the days of Avraham; and Yitzchak went to Avimelech, King of the Philistines, in Gerar." (Bereshit 26:1)
In connection with Yaakov, we find "Arise, therefore, and go to Aram, the house of Bethuel, your mother's father." (Bereshit 28:2)
Similarly, Yosef was sold as a slave to Egypt. Moshe and Aharon led the Jewish People during their forty year sojourn in the desert. And David fled from his enemies into the Desert of Judea.
In all the above cases, we were taught how to maintain our faith in the Protection of Hashem despite great adversity, which is one of the secrets of the survival of the Jewish People.
In some Sephardic Communities, there is a "minhag" or custom, to prepare a special chair, decorated and dedicated in honor of the main guest of the evening, on each of the Seven Nights of Sukkot.