The Nativity of Our Most Holy Lady Theotokos and Ever-Virgin Mary is the first twelfth (from the Slavic "twelve" - twelve) Orthodox feast of the church year. It is the first according to the chronology of events in New Testament history, and according to the church calendar, which begins in September and is therefore called in liturgical hymns "the beginning of our salvation." The birth of the Mother of God was the fulfillment of prophecies that Christ, the Savior of the world, would soon come to earth. The feast of the Nativity of the Most Holy Theotokos is annually celebrated by the Church on September 21 (September 8, old style), is non-transitory and has 1 day of fore-feast and 4 days of after-feast.Source: https://ruvera.ru/rojestvo_bogorodicy