The Feast of Pentecost commemorates the descent of the Holy Spirit upon the Apostles on the day of Pentecost. It also welcomes the establishment of the Church through the preaching of the Apostles and the baptism of thousands of people that day believed in the Gospel message of salvation through Jesus Christ.
After Christ's ascension, the disciples returned to Jerusalem from the Mount of Olives. Over the next 10 days, they're all together in "prayer with the women and Mary the mother of Jesus, and his brothers" (Acts 1:12-14). Prayer is the prerequisite for the Holy Spirit. At some point in our lives we must advance the concerns of this world and God. We have to lock ourselves in the upper chamber of our souls, and pray. While they were all gathered in prayer, the Holy Spirit descended upon them as tongues of fire (Acts 2:3).
Just as Easter or Passover (Pesach), Pentecost was a Jewish holiday. It started as the Harvest Festival, as it is written: "Celebration of the first fruits of the harvest of crops planted in his field. Celebrating the Harvest Festival at the end of the year when they gather in their crops in ground "(Exodus 23:16).
Later, the party became more spiritual, God commemorating the promulgation of the Law to Moses. Christian Pentecost has kept these two aspects. The descent of the Holy Spirit upon the disciples (Acts 2:3), registered in the new law in their hearts the spiritual aspect of the celebration. The "3000 (which) were added to their number that day" (Acts 2:41), and miracles of the early Christians, the first fruits of the new religion of Jesus.
According to Tertullian, Christians celebrated the day of Pentecost in the third century.
In the fourth century, the celebration lasted a week and had equal status with Easter.
