Because you are part of the body of the Church, Pentecost is the Church's birthday, and yours as well. And like any birthday, it's a cause for celebration.
The word Pentecost is Greek and it means "50th day." 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.