The Day of Pentecost, also known as Pentecost Sunday, is a significant feast in the Catholic calendar. It is celebrated 50 days after Easter and marks the descent of the Holy Spirit upon the Apostles and the Virgin Mary, as described in the Acts of the Apostles (Acts 2:1-31). This event empowered the Apostles to begin their mission of spreading the Gospel, making it known as the "birthday of the Church."