One of the least understood concepts of Christianity remains the concept of the "Holy Trinity," a belief that God is present in three distinct forms; namely, God the Father, God the Son and God the Holy Spirit. This belief led early scholars to posit that Christianity was not, as claimed, a monotheistic religion. Fairly complex Catholic theological arguments disputed such claims of polytheism, however, by describing a unique situation in which God presented himself - "Himself" in the Christian context - to people on earth in various forms. As an extension, the split in the Catholic Church that led to various so-called "Protestant" sects led to these offshoots to extend to the concept of the Holy Trinity beyond the Church itself. In the modern-day, there is little to distinguish between the root beliefs of Catholic Christians and Protestant Christians, and the concept of the Holy Trinity is universal among them.
While the concept of Jesus as "God on earth" is fairly well-understood as a Christian tenet even among non-Christians - due largely to the celebrations of Christmas and Easter, which center on the birth, death and resurrection of Jesus Christ - it is the third iteration of God that tends to confuse those not familiar with the concept of the Holy Trinity. The Pentecost, a lesser-known but no less important holy day in the Christian faith, is a celebration of the appearance of the Holy Spirit.
Literally translated, "Pentecost" means "the fiftieth day" and is a holy day that is celebrated seven weeks (or 50 days) after Easter Sunday. The feast, which is especially relevant in the Christian liturgical year, is alternately known as "Whitsun" or "Whitsunday," especially in the United Kingdom. The biblical reference to the Pentecost occurs in the New Testament in the Acts of the Apostles (Chapter 2), and because of the great significance of this moment in the formation of the Catholic Church, the day itself is often referred to as the "Birthday of the Church."
As with Christianity itself, there is a Jewish undertone to this original event. The traditional interpretation of the first Pentecost has the event occurring in the "Upper Room" or "Cenacle" during the Jewish celebration of Shavuot, which commemorates the Ten Commandments being handed down from God to Moses at Mount Sinai fifty days after the Jewish Exodus from Egypt.
There are a wide variety of celebratory traditions associated with the Pentecost in various branches of Christianity, but the underlying significance of the appearance of the Holy Spirit to the disciples of Jesus remains the same throughout. What differs widely, however, is the modern-day emphasis placed on the feast within the various churches. In modern Catholicism, the Pentecost is by definition, secondary to Easter and Christmas, while in the Eastern Orthodox Church, Pentecost is one of the Great Feasts and ranks second in importance only to Easter, the holiday that celebrates the resurrection of Jesus Christ that is at the very heart of the Christian belief system and tradition. Regardless of the modern-day emphasis placed on the holy day in the various sects, Pentecost itself remains central to the Christian concept of the Holy Trinity, and introduces the third iteration of the Trinity in the form of the Spirit.