Pentecost means "fifty" and marks the fiftieth day following Passover if you are Jewish. If you are Christian, it is a celebration of the gift of the Holy Spirit, presented to a group of Jesus' disciples who were waiting together in a house in Jerusalem, and it marks the birth of the church and the Christian religion. It was followed by a strange incident in which Saint Peter addressed a large crowd of foreigners and all understood him as if he was speaking their native language. You can read about it in Acts chapters 1 and 2. Although generally not celebrated by mainstream Christians apart from a mark on a calendar, it is quite possibly the most important event in church history next to the crucifixion and resurrection.