Pentecost commemorates the descent of the Holy Spirit upon the Apostles in Jerusalem. It is a Solemnity in the Roman Catholic Church.
The word comes out of the Greek word Pentekoste Πεντηκοστή, or fiftieth, and originally referred to the Jewish festival celebrated on the fiftieth day after First Fruits. This festival, the festival of Weeks (שבועות ) is celebrated seven weeks and one day after the first day of Passover and can be found in Leviticus 23:16 -
16 Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the Lord.
In the Acts of the Apostles the New Testament account can be found :
2 When the day of Pentecost came, they were all together in one place. 2 Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. 3 They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4 All of them were filled with the Holy Spirit and began to speak in other tongues[a] as the Spirit enabled them.
5 Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. 6 When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken. 7 Utterly amazed, they asked: “Aren’t all these who are speaking Galileans? 8 Then how is it that each of us hears them in our native language? 9 Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia,[b] 10 Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome 11 (both Jews and converts to Judaism); Cretans and Arabs—we hear them declaring the wonders of God in our own tongues!” 12 Amazed and perplexed, they asked one another, “What does this mean?”
13 Some, however, made fun of them and said, “They have had too much wine.”
Peter Addresses the Crowd
14 Then Peter stood up with the Eleven, raised his voice and addressed the crowd: “Fellow Jews and all of you who live in Jerusalem, let me explain this to you; listen carefully to what I say. 15 These people are not drunk, as you suppose. It’s only nine in the morning! 16 No, this is what was spoken by the prophet Joel:
17 “‘In the last days, God says, I will pour out my Spirit on all people. Your sons and daughters will prophesy, your young men will see visions, your old men will dream dreams. 18 Even on my servants, both men and women, I will pour out my Spirit in those days, and they will prophesy. 19 I will show wonders in the heavens above and signs on the earth below, blood and fire and billows of smoke. 20 The sun will be turned to darkness and the moon to blood before the coming of the great and glorious day of the Lord. 21 And everyone who calls on the name of the Lord will be saved.’[c]
22 “Fellow Israelites, listen to this: Jesus of Nazareth was a man accredited by God to you by miracles, wonders and signs, which God did among you through him, as you yourselves know. 23 This man was handed over to you by God’s deliberate plan and foreknowledge; and you, with the help of wicked men,[d] put him to death by nailing him to the cross. 24 But God raised him from the dead, freeing him from the agony of death, because it was impossible for death to keep its hold on him. 25 David said about him:
“‘I saw the Lord always before me. Because he is at my right hand, I will not be shaken. 26 Therefore my heart is glad and my tongue rejoices; my body also will rest in hope, 27 because you will not abandon me to the realm of the dead, you will not let your holy one see decay. 28 You have made known to me the paths of life; you will fill me with joy in your presence.’[e]
29 “Fellow Israelites, I can tell you confidently that the patriarch David died and was buried, and his tomb is here to this day. 30 But he was a prophet and knew that God had promised him on oath that he would place one of his descendants on his throne. 31 Seeing what was to come, he spoke of the resurrection of the Messiah, that he was not abandoned to the realm of the dead, nor did his body see decay. 32 God has raised this Jesus to life, and we are all witnesses of it. 33 Exalted to the right hand of God, he has received from the Father the promised Holy Spirit and has poured out what you now see and hear. 34 For David did not ascend to heaven, and yet he said,
“‘The Lord said to my Lord: “Sit at my right hand 35 until I make your enemies a footstool for your feet.”’[f]
36 “Therefore let all Israel be assured of this: God has made this Jesus, whom you crucified, both Lord and Messiah.”
37 When the people heard this, they were cut to the heart and said to Peter and the other apostles, “Brothers, what shall we do?”
38 Peter replied, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. 39 The promise is for you and your children and for all who are far off—for all whom the Lord our God will call.”
40 With many other words he warned them; and he pleaded with them, “Save yourselves from this corrupt generation.” 41 Those who accepted his message were baptized, and about three thousand were added to their number that day.
The feast of Pentecost is a movable date and occurs seven weeks after Easter Sunday. The earliest possible day is May 10th and the latest possible date is June 13. Pentecost Monday is considered a public holiday in Andorra, Austria, Belgium, Benin, Cyprus, Denmark, France, Germany, Greece, Hungary, Iceland, the Netherlands, Norway and Romania.
The main liturgical color for Pentecost is Red as it symbolizes both joy and the fire of the Holy Spirit. Red banners are often hung from walls to symbolize the gushing wind of the Holy Spirit. Eastertide ends with Pentecost and as much the dismissal with the double alleluia is sung at the end of mass and the Paschal Candle removed from the sanctuary. The sequence hymn for the day is Veni Sancte Spiritus. In some areas of Ital rose petals are thrown from the galleries over the congregation - a modern twist includes origami doves being dropped from the ceiling. A special Pentecost novena is often recited in the days leading up.
In both the Eastern and Western churches the Blessed Virgin Mary is honored for her central role on the divine concession of the gift of the Holy Spirit to the Apostles. Pope Pius XII, in his Mystici Corporis Christi wrote:
She it was through her powerful prayers obtained that the spirit of our Divine Redeemer, already given on the Cross, should be bestowed, accompanied by miraculous gifts, on the newly founded Church at Pentecost; and finally, bearing with courage and confidence the tremendous burden of her sorrows and desolation, she, truly the Queen of Martyrs, more than all the faithful "filled up those things that are wanting of the sufferings of Christ...for His Body, which is the Church"; and she continues to have for the Mystical Body of Christ, born of the pierced Heart of the Savior, the same motherly care and ardent love with which she cherished and fed the Infant Jesus in the crib.
And while speaking in Rome on May 28th, 1997, Saint Pope John Paul II stated:
Retracing the course of the Virgin Mary’s life, the Second Vatican Council recalls her presence in the community waiting for Pentecost. “But since it had pleased God not to manifest solemnly the mystery of the salvation of the human race before he would pour forth the Spirit promised by Christ, we see the Apostles before the day of Pentecost ‘persevering with one mind in prayer with the women and Mary the Mother of Jesus, and with his brethren’ (Acts 1:14), and we also see Mary by her prayers imploring the gift of the Spirit, who had already overshadowed her in the Annunciation” (Lumen gentium, n.59). The first community is the prelude to the birth of the Church; the Blessed Virgin’s presence helps to sketch her definitive features, a fruit of the gift of Pentecost. [...]
In contemplating Mary’s powerful intercession as she waits for the Holy Spirit, Christians of every age have frequently had recourse to her intercession on the long and tiring journey to salvation, in order to receive the gifts of the Paraclete in greater abundance. [...]
In the Church and for the Church, mindful of Jesus’ promise, she waits for Pentecost and implores a multiplicity of gifts for everyone, in accordance with each one's personality and mission.
In North Western England, parades called Whit Walks take place at Whitsun and contain bands and choirs with the girls often dressing in white. Other festivals include Whit Fairs (Whitsun Ales), morris dancing and cheese rolling.