The Nativity of Our Most Holy Lady Theotokos and Ever Virgin Mary is celebrated by the Church as a day of universal joy. Within the context of the Old and the New Testaments, the Most Blessed Virgin Mary was born on this radiant day, having been chosen before the ages by Divine Providence to bring about the Mystery of the Incarnation of the Word of God. She is revealed as the Mother of the Savior of the World, Our Lord Jesus Christ.

Your Nativity, O Virgin, / Has proclaimed joy to the whole universe! / The Sun of Righteousness, Christ our God, / Has shone from You, O Theotokos! / By annulling the curse, / He bestowed a blessing. / By destroying death, He has granted us eternal Life.

https://www.oca.org/saints/lives/2019/09/08/102541-the-nativity-of-our-most-holy-lady-the-mother-of-god-and-ever-vi