Interfaith Days: Nativity of Mary

https://oca.org/saints/lives/2008/09/08/102541-
the-nativity-of-our-most-holy-lady-the-mother-of-god-and-ever-vi

Today is the Nativity of the Blessed Virgin Mary. In Orthodox Christianity, her birth is also celebrated on this day, as the Nativity of the Theotokos. The day is nine months after the solemnity of Mary's Immaculate Conception (December 8th). Her birth is recorded not in the canonical scriptures but in the second-century Infancy Gospel (Protoevangelium) of James, chapter 5, while the feast itself dates from the fifth century. (The text of the James writing can be found here.) Along with John the Baptist, she is one of the few persons whose birth rather than death date are commemorated by the church, because of their special place in the history of salvation.

This site gives information about the Western feast, while this site provides material on the Eastern observance.