The feast day of Pentecost is often called the birthday of the Church. It celebrates the day when the Holy Spirit descends on the Apostles and appears above their heads as a tongue of flame. Pentecost marks the time when the Apostles finally start preaching Jesus' word to all the people of the world. It is a very important day in the Church's liturgical year because it marks the time that Jesus' salvation was made clear to all who would ask for it. For this reason, it is important to understand Pentecost.
Explain that Pentecost came fifty days after the harvest in ancient Israel. It is usually celebrated on the seventh Sunday after Easter Sunday in modern times.
Explain that the Holy Spirit came to the apostles on this day and enabled them to preach Jesus' message to the people of Jerusalem. It enabled them to speak in the languages of many different peoples. That is why Pentecost is often called the birthday of the Church.
Explain that Pentecost is also the day that people fully understand that Jesus died for the sins of people and that salvation can be found through Him. This gift is bestowed by baptism which started on the first Pentecost.