USHPIZIN (Aram. אֻשְׁפִּיזִין; from the Lat. hospes, "guest"), according to kabbalistic tradition, the mystical seven "guests" – Abraham, Isaac, Jacob, Moses, Aaron, Joseph, and David – who visit the sukkah during the seven days of Tabernacles (cf. Zohar, 5:103b). According to the Zohar, Joseph comes after Moses and Aaron, but in most Ashkenazi maḥzorim and prayer books the order is chronological. The spiritual guest of each day is invited before the meal and the text of this invitation, "Enter, exalted holy guests…," is found in several Ashkenazi and Sephardi prayer books. The custom was adopted by the Ḥasidim and many pamphlets entitled Seder-Ushpiz, including liturgy based upon the practices of certain ẓaddikim (e.g., the rabbis of Belz, Zanz, etc.), began to be published in the 19th century. Decorating the sukkah wall with a plaque which bears an inscription including the names of the seven guests has also become an accepted practice. Moroccan Jews have a special compilation of prayers in honor of the ushpizin, called Ḥamad Elohim, from which special sections are recited each day of the festival.
Eisenstein, Dinim, 12ff.