Christ Our Pascha: Pentecost: The Descent of the Holy Spirit
Pentecost: The Descent of the Holy Spirit After The dwelling of the Holy Spirit in the Church was first visibly manifested on Pentecost Sunday, when He came down upon the Apostles in the form of tongues of fire. After the Ascension, the Apostles together with the blessed Virgin and disciples, men and women, numbering about 120 persons, gathered in the Cenacle, the upper room in Jerusalem where the Last Supper had been taken. There they spent the time in prayer, awaiting the fulfillment of Our Lord's promise: "Wait here in the city, until you are clothed with power from on high" (Luke 24:49).
Jesus had promised to send the Holy Spirit to the Apostles. He said at the Last Supper: "It is expedient for you that I depart. For if I do not go, the Advocate will not come to you; but if I go, I will send him to you" (John 16:7) . On Pentecost, ten days after the Ascension, the Holy Spirit descended on the Apostles and disciples. On Pentecost three thousand members were baptized after the preaching of St. Peter. Many believed, because the Apostles had the "gift of tongues' that is, they spoke in one language, but those of different races who listened heard what was said in their own different languages. We celebrate the descent of the Holy Ghost today as Pentecost Sunday, ten days after Ascension Thursday, fifty days after Easter. Pentecost means fifty.
The nine days in the Cenacle while the Apostles and disciples waited for the coming of the Holy Spirit were spent in prayer, the first novena in the Church. "All these with one mind continued steadfastly in prayer with the women and Mary, the mother of Jesus" (Acts 1:14). In imitation of that first novena, it is our custom today to make novenas especially in preparation for great feasts. We also make novenas of petition or thanksgiving.
How long will the Holy Spirit dwell in the Church? --The Holy Spirit will dwell in the Church until the end of time.
"I will ask the Father and he will give you another Advocate to dwell with you forever, the Spirit of truth" (John 14:16-17) The Holy Spirit watches over the Church, protecting it from destruction. From the beginning the Church spread very rapidly. At the death of the Apostles, in spite of persecutions, it was known in all parts of the then civilized world. Thence it has spread to the ends of the earth.
St. Paul could say: "Yes, indeed, their voice has gone forth into all the earth, and their words unto the ends of the world" (Rom. 10:18). The Holy Spirit gave testimony of Christ, and strengthened the Apostles to give testimony of Christ.
Our Lord said, "But when the Advocate has come, whom I will send you from the Father, the Spirit of truth who proceeds from the Father, he will bear witness concerning me. And you also bear witness" (John 15:26,27). By His descent, the Holy Spirit proved that all Jesus Christ had said and done was true, that He was indeed the Son of God. After the coming of the Holy Spirit, the Apostles gave testimony of Christ by going all over the world, (Acts 1:8), preaching and suffering for Christ, meeting death joyfully (Acts 5:41; Rom. 8:18) , saying, "I can do all things in him who strengthens me."
Taken from http://www.catholicbook.com/AgredaCD/MyCatholicFaith/mcfc038.htm