III. And so this fast comes very opportunely after the feast of Whitsuntide
Therefore, after the days of holy gladness, which we have devoted to the honour of the Lord rising from the dead and then ascending into heaven, and after receiving the gift of the Holy Ghost, a fast is ordained as a wholesome and needful practice, so that, if perchance through neglect or disorder even amid the joys of the festival any undue licence has broken out, it may be corrected by the remedy of strict abstinence, which must be the more scrupulously carried out in order that what was on this day Divinely bestowed on the Church may abide in us. For being made the Temple of the Holy Ghost, and watered with a greater supply than ever of the Divine Stream, we ought not to be conquered by any lusts nor held in possession by any vices in order that the habitation of Divine power may be stained with no pollution.
Source: Sermons (New Advent)