We find the celebration of Pentecost in the Old Testament. It was observed as a “festival of first fruits,” a time to thank God for the harvest. We usually don’t think of this time of the year as a harvest season, but in milder climates, like the Mediterranean, it is so.
The Jewish people celebrated this festival, called Shavout fifty days after Passover. Pentecost is derived from the Greek word for fifty. In time it also became known as the “feast of weeks” (seven weeks plus one day after Passover) and celebrated God’s covenant with the people made on Mount Sinai when God gave Moses the Law.
Christians celebrate Pentecost because it was on this feast that the Holy Spirit descended upon the apostles. It is an event recounted in the Acts of the Apostles. It concludes the Paschal season—50 days after Easter—and is considered the “Birthday of the Church.”