Pentecost Sunday marks the birth of the Catholic Church! The day the Holy Ghost descended upon the Apostles in the Upper Room (Acts 2:1-12). From this event forward, the Apostles began their "Great Commission" (Matt. 28:19-20; Mark 16:15; Luke 14:23) to "go forth into the whole world making disciples of all nations..." with a newfound courage and fury. They came out from the Upper Room speaking the languages of all who were there for the Jewish celebration of Pentecost. Jews were in Jerusalem from all around the world for this pilgrimage and they each heard the Apostles preaching in their own languages! Truly remarkable for a bunch of men who never left the Palestine area.
This feast day is celebrated in both the Eastern and Latin Churches in much the same way:
The holiday - or "holy day" - actually is rooted in Jewish tradition. The Festival of Weeks, or Shavuot, which was originally a harvest festival. It was to be celebrated fifty days after the celebration of unleavened bread (Passover) when a new grain offering was to be made.
We get the name "Pentecost" from Greek speaking Jews who used the term "pentekostos" for the fiftieth day.
While many other Christian churches/communities celebrate Pentecost NONE can claim the direct apostolic succession which can be found in the Catholic and Orthodox Christian Churches. Most non-Catholic Christian groups were born sometime after 1517AD (October 31, 1517 is when Luther nailed the 95 Thesis to the door of the Catholic Church in Wittenberg and most Protestants view this as the birth of Protestantism) or shortly thereafter when King Henry VIII separated the Catholic Church of England from the One, Holy, Catholic and Apostolic Church. Protestantism took Europe by storm though several bastions of Catholicism remained and still remain to this day. The point of irony here is that if Catholicism is false then Jesus would have waited over 1500 years to build His Church, as He promised He would do (Matthew 16:18-19). He also promised the Paraclete, the Holy Ghost, would come to His Church and stay with His Church until He returned again in glory (John 14:15-19). Well again, on Pentecost Sunday we celebrate the descent of the Holy Ghost upon the Blessed Virgin and the Apostles. So, either Catholicism is true - or Jesus was a liar.