Q. 1. How did the Catholic Church determine that the birth of Mary took place on September 8?

A. 1. The Feast of the Nativity of the Blessed Virgin Mary is celebrated on September 8 th because it is exactly nine months after the Feast of the Immaculate Conception on December 8 th. If Mary was conceived on December 8 th, then she had to have been born on September 8 th.