The feast of the Nativity of the Blessed Virgin Mary is celebrated each year on September 8, a Marian Feast that dates back to the sixth century. It is generally believed that the date of September 8 was chosen to celebrate the Nativity of Mary since the civil year began in Constantinople on September 1. Scholars believe that this date was chosen since it was symbolic that the "beginning" of the work of salvation should be commemorated near to the beginning of the new year. The Solemnity of the Immaculate Conception of Mary was later fixed at December 8, nine months prior.
Learn more at https://udayton.edu/imri/mary/b/birth-of-mary-september-8.php