In the Catholic Church, we celebrate the death of saints as the day they overcame the world with all its temptations and became united with God in heaven. All saints are commemorated on the day of their passing from this world because this is the day they attained perfection. The day they became united with God.
However, the church celebrates the birthday of only three people; Jesus, Mary and St. John the Baptist.
The birth of Jesus is celebrated because Jesus is God himself. The second person of the trinity. He is holiness personified.
The birth of Jesus is also celebrated because it is a joyful day for mankind. It is the day God came into the world with the sole purpose of redeeming us. The birth of Jesus is the fulfilment of God’s promise to send us a saviour. The birth of Jesus is indeed a joyful day; it is celebrated all around the world on the 25th of December as Christmas.
The birth of the Blessed Virgin Mary is also celebrated because she is the Immaculate Conception; she was conceived without sin. She was specially created by God for the sole purpose of being the co-redemptrix of our salvation. She remained sinless from conception till her assumption into heaven. Unlike the saints who were united to God at their death, the Blessed Virgin Mary was united to God from conception and is still united with Him. This is why her birthday is celebrated all through the Catholic Church on the 8th of September of every year.
The birthday of John the Baptist is celebrated because while he was still in the womb of his mother, he encountered Jesus. This encounter was so great that he leapt for joy in the womb of his mother Elizabeth. This encounter it is believed, wiped away any stain of original sin he may have had. This means that John the Baptist came into this world sinless; he was born without any stain of original sin. That early encounter with Jesus in the womb had the same encounter that baptism would ordinarily have. It wiped away the stain of original sin on john the Baptist; so that from the day of his birth till his death, he was united with God. He was holy. His birthday is celebrated all through the Catholic Church on the 24th of June