Consider what our prospects were before the birth not only of Our Lord, but from before the birth of His Mother, from whom He took our human nature, the Blessed Virgin Mary.
Today’s feast is older than the Feast of the Immaculate Conception, which was precisely nine months ago. Holy Church, in celebrating liturgically her holy birth for a long time, ultimately reasoned back to Mary’s holy conception. As St. Thomas Aquinas argued,
“The Church celebrates the feast of our Lady’s Nativity. Now the Church does not celebrate feasts except of those who are holy. Therefore, even in her birth the Blessed Virgin was holy. Therefore, she was sanctified in the womb.” (STh III, q. 27, a. 1)
Here is the entry in the Roman Martyrology for today’s feast of the Nativity of the Blessed Virgin Mary:
Festum Nativitatis beatae Mariae Virginis, ex semine Abrahae, de tribu Iuda ortae, ex progenie regis David, e qua Filius Dei natus est, factus homo de Spiritu Sancto, ut homines vetusta servitute peccati liberaret.
The feast of the Nativity of the Blessed Virgin Mary, sprung from the seed of Abraham [and] from the tribe of Judah, from the line of David the king, from which was born the Son of God, made man of the Holy Ghost, that he might free men from the ancient slavery to sin.