Catholic Culture's Pentecost entry:
Pentecost (Whitsunday), with Christmas and Easter, ranks among the great feasts of Christianity. It commemorates not only the descent of the Holy Spirit upon the Apostles and Disciples, but also the fruits and effects of that event: the completion of the work of redemption, the fullness of grace for the Church and its children, and the gift of faith for all nations.
After Jesus had ascended to heaven from Mt. Olivet, the apostles and disciples returned to the Holy City. They remained together in the Upper Room or Cenacle, the place where Jesus had appeared to them and which may well be called the first Christian church. About a hundred and twenty persons were assembled there. They chose Matthias as an apostle in place of the unhappy Judas; they prayed and waited for the Paraclete.
Ten days had passed, it was Sunday, the seventh Sunday after the resurrection. At about nine o'clock in the morning, as they were together praying fervently, the Holy Spirit descended upon them. Note how all the great theophanies in Christ's life occurred during the course of prayer. After His baptism, for instance, when Jesus was praying the heavens opened and the Holy Spirit descended in the form of a dove; likewise, it was during prayer at night that the transfiguration took place on Tabor.
Surely too it was while Mary was praying that Gabriel delivered his message, and the Holy Spirit overshadowed her. Pentecost followed precedent. The small community of Christians had prepared themselves through prayer for the coming of the Paraclete. The same is true at Mass today, every day; through prayer we ready our souls for the advent of the Spirit.
The descent upon the apostles was internal and invisible in nature although accompanied by certain visible phenomena. There came a mighty roar, like the onrush of a violent wind. It came suddenly, from heaven; but unlike storms that strike a structure from without, this one penetrated and filled the room where the disciples were gathered. Therefore it was not a natural wind, it was a miracle peculiar to the occasion. A second visible sign consisted in tongues of fire that descended upon each one present. These fiery tongues gave visible evidence that the Holy Spirit had descended upon them.
Today at Mass, particularly at holy Communion, the power of the Holy Spirit will come down upon us; fiery tongues will not be seen, but invisible tongues of fire will not be absent. There was still another external manifestation of the Holy Spirit; the apostles and disciples were enabled to speak various languages.
After the roar of the wind many of Jerusalem's pilgrims hurried to the Cenacle. Pentecost was one of the three festivals which obliged all Jews to be present in Jerusalem. Jews from distant lands, and Jewish converts from paganism too, attended these feasts. As a result, a colorful crowd speaking a variety of languages surrounded the house. Now the apostles, who so shortly before had hid in fear behind locked doors, came forth and courageously walked among the multitude speaking to each in his native tongue. It was indeed amazing! Galileans, and multilingual?