Abstract: The subject of this research is the structure of the service devoted to the fifth Saturday of the Great Lent (the Saturday Akathist). This is a unique service because it is when Akathist, the ancient hymn in the honor of the Holy Mother that relates to the Feast of the Annunciation, is being read. The object of the research is a series of manuscripts of Slavonic Triodions of the 12th - 14th centuries. In his research Vorobiev presents and analyzes data from nine manuscripts. Eight manuscripts were studied de visu, and a fascimile edition of one manuscript was studied. The main research method used by the author is the typological analysis that allowed to define similiarity between the present Saturday Akathist service and the services described by Greek sources that described the procedure of the Byzantine service. The results of the research demonstrate that the Saturday Akathist service was often compiled on oe of famous West Byzantine liturgical models. Such compilation can be proved by both typological and textological features. This conclusion made by the author creates a new understanding of the process of creating and changing the Slavonic-based Triodion liturgical books.