Among the Jews this was a harvest feast and was celebrated about seven weeks after the Passover. Later it commemorated the promulgation of the Ten Commandments on Mount Sinai.
The Christian feast is on the seventh Sunday after Easter and commemorates the descent of the Holy Spirit upon the apostles in the Cenacle. It is a "harvest-feast" of souls, since on that day the Holy Spirit empowered the Apostles, removed their fear, gave them the power of being heard in many different languages, and many thousands were baptised on that day. Pentecost is 50 days after Easter ("Pente" meaning 50)
When the day of Pentecost had come, they were all together in one place. And suddenly from heaven there came a sound like the rush of a violent wind, and it filled the entire house where they were sitting.
Divided tongues, as of fire, appeared among them, and a tongue rested on each of them.
All of them were filled with the Holy Spirit and began to speak in other languages, as the Spirit gave them ability.
The Descent of the Holy Spirit is the third of the Glorious Mysteries of the Rosary.
Today is recognised as the great beginning of the Church.