Why is Pentecost called the birthday of the Church?

Pentecost is known as the "birthday" of the Catholic Church, as it marks the outpouring of the Holy Spirit upon the apostles.

Everyone has a birthday, and Pentecost marks the official birthday of the Catholic Church.

The reason behind this is that Pentecost completes the paschal mystery, as the Catechism of the Catholic Church explains.

On the day of Pentecost when the seven weeks of Easter had come to an end, Christ’s Passover is fulfilled in the outpouring of the Holy Spirit, manifested, given, and communicated as a divine person: of his fullness, Christ, the Lord, pours out the Spirit in abundance.
On that day, the Holy Trinity is fully revealed. Since that day, the Kingdom announced by Christ has been open to those who believe in him: in the humility of the flesh and in faith, they already share in the communion of the Holy Trinity. By his coming, which never ceases, the Holy Spirit causes the world to enter into the “last days,” the time of the Church, the Kingdom already inherited though not yet consummated.
CCC 731-732

Pentecost is the day when the Church’s sacraments are inaugurated, specifically the sacrament of baptism.

Because the Holy Spirit is the anointing of Christ, it is Christ who, as the head of the Body, pours out the Spirit among his members to nourish, heal, and organize them in their mutual functions, to give them life, send them to bear witness, and associate them to his self-offering to the Father and to his intercession for the whole world. Through the Church’s sacraments, Christ communicates his Holy and sanctifying Spirit to the members of his Body.
CCC 739

The Church can now fulfill her mission, with the help and assistance of the Holy Spirit.

St. Peter wasted no time on this birthday of the Church and proceeded to baptize 3,000 individuals!

Pentecost is a beautiful day in the Church’s calendar, a day that many families celebrate with a birthday cake.

Happy Birthday, Church!