On the day Jesus ascended into Heaven, He told His disciples to wait in Jerselum for the Holy Spirit to come to them, and help them be His witnesses to the ends of the Earth. Read the Bible account in Acts Chapter 1.

Nine days later, on the day of Pentecost, the feast of the coming of the Law, the Holy Spirit descended on the disciples--including Peter and the other apostles, Mary, and others--who were gathered in prayer. The Spirit came in a form like fire and with a sound like a mighty wind. Peter and others began to preach to people gathered in Jeruselum from all over the world, telling them about Jesus and exorting them to repent and be baptized. Each person there heard the apostles speaking in his or her own language. About 3, 000 people were baptized that day. Read the Bible account in Acts Chapter 2.
 

See Scripture Readings for Pentecost Mass HERE.

O God, who by the mystery of today's great feast sanctify your whole Church in every people and nation, pour out, we pray, the gifts of the Holy Spirit across the face of the earth and, with the divine grace that was at work when the Gospel was first proclaimed, fill now once more the hearts of believers.

Through our Lord Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, God, for ever and ever. Amen.

(The collect is an opening prayer, prayed out loud by the priest, that gathers together the prayers of the people at the end of the introductory rites of the Mass. The collect reflects the day's feast. (Youtube video explaining the collect is linked HERE. (YouTube link. There may be ads.)
 

 Come, Holy Spirit, come!
And from your celestial home
    Shed a ray of light divine!
Come, Father of the poor!
Come, source of all our store!
    Come, within our bosoms shine.
You, of comforters the best;
You, the soul’s most welcome guest;
    Sweet refreshment here below;
In our labor, rest most sweet;
Grateful coolness in the heat;
    Solace in the midst of woe.
O most blessed Light divine,
Shine within these hearts of yours,
    And our inmost being fill!
Where you are not, we have naught,
Nothing good in deed or thought,
    Nothing free from taint of ill.
Heal our wounds, our strength renew;
On our dryness pour your dew;
    Wash the stains of guilt away:
Bend the stubborn heart and will;
Melt the frozen, warm the chill;
    Guide the steps that go astray.
On the faithful, who adore
And confess you, evermore
    In your sevenfold gift descend;
Give them virtue’s sure reward;
Give them your salvation, Lord;
    Give them joys that never end. Amen.  Alleluia.