Final answer:
The Holy Spirit descended upon the apostles on the day of Pentecost, empowering them to preach the Christian gospel in various languages.
Step-by-step explanation:
The Holy Spirit came upon the apostles on the day of Pentecost. This event, depicted in the biblical book of Acts 2, occurred fifty days after Pascha (Easter) and is commemorated by Christians as the moment when the apostles received the Holy Spirit in the form of tongues of fire and began to preach in different languages.
The descent of the Holy Spirit empowered the apostles to spread the message of the crucified and risen Christ to various tribes and tongues, signaling the beginning of the Christian Church's mission to the world.