Revelation 7:1-10, 21:1-4
Psalms 113-118 (all week)
The four species are also held and waved during the Hallel prayer in religious services, and are held during processions around the bimah (the pedestal where the Torah is read) called hakafot each day during the holiday. These processions commemorate similar processions around the altar of the ancient Temple in Jerusalem. This part of the service is known as Hoshanot, because while the procession is made, we recite a prayer with the refrain, "Hosha na!" (please save us!). On the seventh day of Sukkot, seven circuits are made. For this reason, the seventh day of Sukkot is known as Hoshanah Rabbah (the great Hoshanah).
From Judaism 101
The sukkah is called by our sages, "refuge of faith." As is the shelter that comes from faith and trust in G-d, and every Israelite may be entitled to this by dwelling in the sukkah.
-Rabbi Eliyahu Kitov