The Synaxis of the Most Holy Theotokos: On the second day of the feast, the Synaxis of the Most Holy Theotokos is celebrated. Combining the hymns of the Nativity with those celebrating the Mother of God, the Church points to Mary as the one through whom the Incarnation was made possible. His humanity – concretely and historically – is the humanity He received from Mary. His body is, first of all, her body. His life is her life. This feast, the assembly in honor of the Theotokos, is probably the most ancient feast of Mary in the Christian tradition, the very beginning of her veneration by the Church.
By Fr. Alexander Schmemann, The Services of Christmas