When the time for Pentecost was fulfilled...Pentecost was the Greek name for a Jewish feast that was celebrated on the fiftieth day after the Passover feast. It was a celebration that commemorated the giving of the Law by God to Moses on Mount Sinai. It was a day worth commemorating because it was on this day that the nation of Israel entered into a special relationship (covenant) with God. It was on this day that God established the nation of Israel as God's chosen, setting her as a light to all the nations of the world. As such, Pentecost was to be observed as a solemnity by the members of the Jewish community. The events about which we have read in today’s First reading took place as the disciples gathered to observe this great Jewish feast. 
However, when Christians throughout the world gather today to celebrate the feast of Pentecost, they do so, not in remembrance of the giving of the Law on Mount Sinai, but rather in commemoration of what took place some two thousand years ago as the disciples gathered in Jerusalem to celebrate the Jewish Pentecost. According to the account in the Acts of Apostles, “ [The apostles] were all in one place together…when suddenly… a noise like a strong driving wind filled the entire house and what appeared as tongues of fire descended and rested on each of them, filling them with the Holy Spirit and they began to speak in different languages as the Spirit enabled them to proclaim” (cf. Acts 2:2-4). For Christians, it is this descent of the Holy Spirit upon the disciples (and by extension upon the Church) that the feast of Pentecost celebrates. It is an important feast for the Christian faithful because it celebrates the birth of the Church. Just like the Jewish Pentecost, the Christian Pentecost celebrates the forming of a new community by God – a community that is being reconciled to God in the Holy Spirit. 
The feast of Pentecost commemorates the re-creation of the universe by God in the Holy Spirit. Today we celebrate God gathering the scattered men and women of the earth and forming them into a new people, a people who can bear witness, make present, and spread the mystery of our communion with God (cf. CCC 738). This is clearly illustrated by the events that followed the outpouring of the Holy Spirit upon the apostles. For as the Holy Spirit descended on the gathered apostles, they began to speak in different tongues, a spectacle that did not pass unnoticed: “ Are not all these people who are speaking Galileans? Then how does each of us hear them in his native language?” It was a spectacle and a shocker at the same time. It was not expected that a group of Jews who had spent their lives exclusively in or around Judaea to speak the language of a people who were a half-world away from them. To understand what was going on and why those who watched the spectacle were amazed, we have to refer to the Genesis story of the tower of Babel (cf. Genesis 11:1-9).
According to the Babel story, the people of the world came up with a plan to build a tower that would take them to God’s abode (euphemism for egotism, pride, arrogance, and strive for independence from God that came to characterize the human person after the fall).  It was a plot that did not sit well with God who in turn decided to halt their ill-conceived plan by confusing their languages and scattering them all over the world. It is a story that attempts to give an explanation for the chaos and feelings of alienation that has come to characterize our world. Because of indifference, competition and fear of the other, the peoples of the world have ended up grouping themselves regionally. These regions – to the most cases distinguished by language - have become exclusive and competitive. An individual from one region desiring to speak the language of another region is always looked upon with suspicion and treated with caution. In addition, there is always hesitancy when it comes to learning about the language/culture of another region. One can therefore imagine the surprise that registered on the faces of the visiting pilgrims when they heard a people who were not from their region speaking in their own tongues. They were surprised because regional grouping of people has led to alienation and distrust of the other. Men and women have ended up looking at those who are different from them with suspicion and distrust. Language that was given men and women as a means of coming together in cooperation has ended up being a means of alienation. 
The Holy Spirit that descended upon the apostles and whose mission we celebrate today breaks the barrier that separates the peoples of the world. It re-creates the condition that existed before the confusion of the world’s languages. The Holy Spirit that continues to descend upon the Church even today seeks to make the whole world to once again “speak the same language,” the language of love, tolerance, understanding, and bearing with one another The Holy Spirit is being given us to transform us into one family of God, brothers and sisters of one another. As it was with the nation of Israel when the Law was given to them, the Holy Spirit enables us to enter into a special relationship with God and one another. It is this very fact that makes Pentecost a special day in the life of the Church. 
-  Ndugu Jeff -