Fifty days after Easter Sunday, we celebrate the coming of the Holy Spirit upon the Apostles and their followers, and the beginning of their Earthly ministry to make disciples of all nations.

Pentecost is also a Jewish holiday, which the Jews use to celebrate the end of Passover. Jews celebrate the gift of the law to Moses at Mt. Sinai on this day. But we, as Catholics celebrate the birth of our Church.

At Pentecost, the Apostles and their followers were gathered in a room. Jews from all over the world were gathered with Peter, the leader of the Apostles and the Eleven. At this time, a great wind blew and a flame appeared as a tongue of fire, which split itself into many individual flames above the heads of all those present. The Holy Spirit came upon these people and each began to speak in tongues. Despite the fact many had no common language, they were perfectly able to understand one another.

Others who were not so blessed, accused those speaking in tongues of being drunk, but Peter arose and addressed the crowd, explaining that it was only 9 o'clock, and that this phenomenon was not intoxication, but rather this was the work of the Holy Spirit, prophesized in the scripture.

Peter then called all those present to be baptized and about three thousand people were baptized that day.

These people were among the first Catholics, and Peter is the first pope of the Catholic Church.

Source: Catholic Online