The Church celebrates plenty feast days of the saints’ throughout the liturgical year, and generally, the date marks the day the saint died and entered into eternity. ( A notable exception is the feast of St. John Paul II, celebrated each October 22, the anniversary of his election as Pope.)
However, the Church celebrates three birthdays in the liturgy:
The birth of Jesus (December 25)
The birth of Mary (September 8)
And the birth of John the Baptist (June 24)
Obviously, the birth of Jesus is a central feast for Catholics, as it marks the day the world came to behold the face of God-made-man.
It is a feast that was complemented by the one nine months earlier, during the Annunciation on March 25, when we celebrate the moment the Incarnation indeed happened, when Jesus was conceived in the womb of Mary.
The celebration of the birthday of Our Lady is also a treasure for Catholics. How could we neglect to celebrate his Mother’s birthday?
Just as Christmas is linked to the feast nine months before, so also the September 8 which is the celebration of the Virgin’s birthday points nine months back, to December 8 is when we celebrate that, from the moment of Mary’s conception in the womb of her mother,
St. Anne, Mary is without sin. She is the Immaculate Conception.
Finally, the birthday of John the Baptist is also rich in symbolism. Of course, John was a special beneficiary, alongside with his mother and of Mary’s care (and Baby Jesus was right there with her!).
As Mary was freshly pregnant with the Child conceived of the Holy Spirit, “She set out and traveled to the hill country in haste to a town of Judah, where she entered the house of Zechariah and greeted Elizabeth.
When Elizabeth heard Mary’s greeting, the infant (that is, John the Baptist) leaped in her womb.”
The tradition of the Church holds that in the prenatal meeting of these tiny cousins, John was filled with the Holy Spirit and then he was born already purified of original sin. ( as we see in Luke 1:15 how the Angel Gabriel told John’s father, Zariah, this would happen: “He will be filled with the holy Spirit even from his mother’s womb.”)
So in that case, John was born without original sin, and of he course went on to have a key role in the mission of Jesus.
As Gabriel told Zechariah, John would “prepare a people who are fit for the Lord.”