The Sunday of the Holy Cross, which is the Third Sunday of Great Lent, was a special celebration in Constantinople, as recorded in a manuscript from 1437 describing the order (diataxis) by which the emperor was to participate in the ceremony. According to this manuscript, after Great Vespers on Saturday evening, the emperor and the patriarch went on procession from Hagia Sophia to the palace chapel of the treasury (presumably the Chapel of Saint Stephen) in which the reliquary of the True Cross was kept. Banners and liturgical fans accompanied them, while a double file of soldiers bearing spears dipped them in homage as the procession passed by. At the chapel, the emperor retrieved the relic and it was borne in its reliquary by both the emperor and the patriarch for the return journey to Hagia Sophia. Arriving at the Great Church of Hagia Sophia, the emperor and patriarch placed the reliquary on the Holy Altar.
The next morning, the emperor returned with his entourage, entering Hagia Sophia from the atrium towards the end of the Canon of Matins, when he is greeted with acclamations from the choir. After venerating the icons he took his seat on his throne. The patriarch entered the church in the same way, accompanied by his metropolitans. They too venerated the icons and, after blessing the people, they took their seats in the Holy Sanctuary.
The preparation for the veneration of the cross began after the Great Doxology, before the Divine Liturgy. The emperor descended from his throne and entered the sanctuary without his crown. In the sanctuary he put on the sticharion and the orarion, thus dressing as a deacon. He then carried the relic upon his head to its position in the solea. There the patriarch performed the ceremony of the Veneration of the Holy Cross.
What we learn from this was that the ceremony was both imperial and liturgical, and clearly articulates the symphonia and complimentary functions between the emperor and patriarch, the Church and the State, in the Roman Empire, showing that they both share an equal burden by carrying the cross on behalf of both.
Significant is the fact that the emperor dressed as a deacon with the sticharion and orarion. According to Pseudo-Kodinos and Symeon of Thessaloniki, the emperor would usually dress as a depotatos in ecclesiastical ceremonies, which is the lowest rank of the ecclesiastical hierarchy corresponding to a subdeacon, wearing a golden mantle over his imperial regalia and carrying a staff in one hand and a cross in the other, and dressed like this he led the Great Entrance. The office of deacon certainly outranks that of a depotatos, but by dressing him as a deacon it also shows that the emperor's priestly charisma or function was the lowest of the three major clerical orders. Even when the depotatos would receive communion at the sanctuary, he did so in the same manner as a deacon to show that liturgically he did not have a higher rank than the priests or bishops. The significance for his dressing like a deacon on this day probably stems from the fact that he is associated with the angels, who live to serve Christ the Great High Priest. It is a symbolic liturgical move in this late period of the empire to make clear the exalted rank of a bishop in the ecclesiastical atmosphere over that of the emperor, who held a quasi-sacredotal liturgical privilege.