Final answer:
Pentecost is the liturgical season celebrating the church's birthday, commemorating the Holy Spirit's descent upon the Apostles, enabling them to speak in diverse languages.
Step-by-step explanation:
The liturgical season that celebrates the birthday of the church is called Pentecost. Pentecost occurs fifty days after Easter and commemorates the descent of the Holy Spirit upon the Apostles as detailed in the Book of Acts, chapter 2. This event marks the beginning of the church's mission in the world and is symbolized by the Apostles speaking in different languages, enabled by the Holy Spirit, which is often artistically represented by tongues of fire descending upon them.