This Synaxis – which is to say, our gathering to honor the Theotokos – is observed in her honor because she miraculously gave birth to the Son and Word of God, becoming the means of mankind’s salvation.
Apolytikion of Synaxis of the Theotokos
Your Nativity, O Christ our God, has brought the light of knowledge to the world; as those who worshipped the stars were led by a star to worship You, the Sun of Righteousness, and to recognize You, the Dayspring from on high. O Lord, we glorify You.
Kontakion of Synaxis of the Theotokos
He who was begotten of the Father before the morning star, without a mother, is now incarnate through you, without a father. Thus, a star proclaims the good news to the Magi. Angels and shepherds praise your pure birth, O Full of Grace.
Source: Greek Orthodox Archdiocese of America