V. The Spirit's work is still continued in the Church
By these and other numberless proofs, dearly-beloved, with which the authority of the Divine utterances is ablaze, let us with one mind be incited to pay reverence to Whitsuntide, exulting in honour of the Holy Ghost, through Whom the whole catholic Church is sanctified, and every rational soul quickened; Who is the Inspirer of the Faith, the Teacher of Knowledge, the Fount of Love, the Seal of Chastity, and the Cause of all Power. Let the minds of the faithful rejoice, that throughout the world One God, Father, Son, and Holy Ghost, is praised by the confession of all tongues, and that that sign of His Presence, which appeared in the likeness of fire, is still perpetuated in His work and gift.
For the Spirit of Truth Himself makes the house of His glory shine with the brightness of His light, and will have nothing dark nor lukewarm in His temple. And it is through His aid and teaching also that the purification of fasts and alms has been established among us. For this venerable day is followed by a most wholesome practice, which all the saints have ever found most profitable to them, and to the diligent observance of which we exhort you with a shepherd's care, to the end that if any blemish has been contracted in the days just passed through heedless negligence, it may be atoned for by the discipline of fasting and corrected by pious devotion.
On Wednesday and Friday, therefore, let us fast, and on Saturday for this very purpose keep vigil with accustomed devotion, through Jesus Christ our Lord, Who with the Father and the Holy Ghost lives and reigns for ever and ever. Amen.
Source: Sermons (New Advent)