the requirement is waived in case of drought. Every day, a blessing is recited over the Lulav and the Etrog. Keeping of Sukkot is detailed in the Hebrew Bible (, and ); the Mishnah (Sukkah 1:1–5:8); the Tosefta (Sukkah 1:1–4:28); and the Jerusalem Talmud (Sukkah 1a–) and Babylonian Talmud (Sukkah 2a–56b). Dwelling in Sukkah The sukkah walls can be constructed of any material that blocks wind (wood, canvas, aluminum siding, sheets). The walls can be free-standing or include the sides of a building or porch. There must be at least two and a partial wall. The roof must be of organic material, known as s'chach, such as leafy tree overgrowth, schach mats or palm fronds – plant material that is no longer connected with the earth. It is customary to decorate the interior of the sukkah with hanging decorations of the four species as well as with attractive artwork. Prayers Prayers during Sukkot include the reading of the Torah every day, reciting the Mussaf (additional) service after morning prayers, reciting Hallel, and adding special additions to the Amidah and Grace after Meals. In addition, the service includes rituals involving the Four Species. The lulav and etrog are not used on the Sabbath. Hoshanot On each day of the festival, worshippers walk around the synagogue carrying the Four Species while reciting special prayers known as Hoshanot. This takes place either after the morning's Torah reading or at the end of Mussaf. This ceremony commemorates the willow ceremony at the Temple in Jerusalem, in which willow branches were piled beside the altar with worshippers parading around the altar reciting prayers. Ushpizin and Ushpizata A custom originating with Lurianic Kabbalah is to recite the ushpizin prayer to "invite" one of seven "exalted guests" into the sukkah. These ushpizin (Aramaic 'guests'), represent the "seven shepherds of Israel": Abraham, Isaac, Jacob, Moses, Aaron, Joseph and David, each of whom correlate with one of the seven lower Sephirot (this is why Joseph, associated with Yesod, follows Moses and Aaron, associated with Netzach and Hod respectively, even though he precedes them in the narrative). According to tradition, each night a different guest enters the sukkah followed by the other six. Each of the ushpizin has a unique lesson to teach that parallels the spiritual focus of the day on which they visit, based on the Sephirah associated with that character. Some streams of Reconstructionist Judaism also recognize