According to the Gospel of Luke 1:26-38, the Archangel Gabriel appeared to Mary to announce to her that she would conceive and bear a son, even though she “knew no man.”
This date was selected by the Church Fathers to be exactly nine months ahead of the Nativity of Our Lord (or vice-versa?), indicating that Christ was conceived in perfection at that time “of the Holy Spirit and the Virgin Mary,” as stated in the Nicene-Constantinopolitan Creed.”
The fast is relaxed today, so for me that means olive oil and wine in my potluck dishes tonight. Recipes coming tomorrow.