The month of April (Overview - Calendar) is dedicated to the Holy Spirit. To believe in the Holy Spirit is to profess that the Holy Spirit is one of the persons of the Holy Trinity, consubstantial with the Father and the Son: "with the Father and the Son he is worshipped and glorified." (Catechism of the Catholic Church)
The Holy Spirit is the Person of Love in the life of God. He is also like a breath, an aspiration of infinite Love, from which we draw the breath of life.
On the day of Pentecost the Divine Spirit communicated such an abundance of life to the whole Church that to symbolize it "there came a sound from heaven, as of a violent wind coming, and it filled the whole house where they (the Apostles) were sitting."
But it is also for us that the Holy Spirit has come, for the group in the Cenacle represented the whole Church. The Holy Spirit came to remain with the Church forever. This is the promise of Jesus Himself. He dwells in the Church permanently and unfailingly, performing in it without ceasing His action of life-giving and sanctification. He establishes the Church infallibly in the truth. It is He Who makes the Church blossom forth with a marvelous supernatural fruitfulness, for He brings to life and full fruition in Virgins, Martyrs, and Confessors those heroic virtues which are one of the marks of true sanctity.
(The Mysteries of the Rosary, Dom Columba Marmion, O.S.B.)