Historically, the Pentecost is considered to be the birth date of the Christian church. Despite having a small number of followers prior to the Pentecost, it was on this day that the Apostles are said to have witnessed the Holy Trinity and begun effectively preaching and adding members to the Christian church.
The Pentecost was originally a Jewish celebration of the day on which God gave the 10 commandments to Moses on Mount Sinai, as well as a general celebration of the early wheat harvest. It was celebrated yearly 50 days after Passover and was referred to as the Feast of Harvest or the Feast of Weeks.
According to scripture, prior to his death, Jesus had told his disciples that 50 days after his ascension, the Holy Ghost would visit and empower them. As was promised, the Holy Ghost came upon the Apostles, inspiring them to officially found the Christian church. They joined the festival celebrations, and Peter, the first Pope, was said to have delivered the first sermon, educating the attendees of the festival on Jesus' death and resurrection and encouraging followers to be baptized. On that initial Pentecost, the first 3,000 members were added to the church.
In orthodox denominations, the Pentecost is now considered to be a celebration of the end of Easter season, 50 days after Jesus' resurrection, as well as a celebration of the unity of the Holy Trinity. It is considered to be an apocalyptic day, marking the final revelation, as well as a celebration of what is to come.