Kalender Camii seems to have been the church of the Monastery of Christ Akataleptos (Incomprehensible) rather than that of the Theotocos Diaconissa (as was thought for a long time). Architecturally, it belongs to the second half of the 9th century.
The drum with 16 windows of the cupola is still rather low, and the large arcades that frame the windows on the outside along the whole height of the walls belong to the tradition of Saint Sophia’s and Saint Irene’s. The ground plan is characteristic of one of the transitional phases between the domed basilica and the church on the inscribed Greek-cross plan. The central cupola is supported by four barrel vaults inscribed, on the outside, beneath triangular gables, but it rests on the walls that divide the four corner compartments. The latter were surmounted by galleries probably reached by outside stairs. The slight dissymetry of the plan is symptomatic of a tentative period.
The church was transformed into a mosque by Mohammed II and restored towards the middle of the 18th century and again in 1855. The three apses of the chevet were demolished and replaced by a straight wall. Parts of the Byzantine marble facings have been preserved. Embedded in the pillars on either side of the old apse are fragments of the carved choir screen which must have dated from the end of the 13th century or the beginning of the 14th; to the left, the Deisis (Christ with the Virgin and St John the Baptist who are interceding for mankind), to the right, the Hetimasie (the Throne of the Last Judgement between two angels).