As far as can be determined,1. Leo delivered these sermons on Pentecost Sunday, with the exception of Serm. 80, which he gave on one of the fast days. Serms. 75-77, while barely mentioning the fast, emphasize the Feast of Pentecost, where Leo describes the coming of the Holy Spirit upon the apostles ( Serm. 77.1).
As usual, Leo denounces the relevant heresies. Macedonianism, while holding the Father and the Son to be equal, considered the Holy Spirit to have an inferior nature ( Serm. 75.4), thus undermining the Trinity, for "by no reckoning is that truly one which is different by any inequality" ( Serm. 77.3). Leo decries the claim that the Manichaeans make concerning Mani and the Holy Spirit ( Serm. 76.2). He lists the marks of the Holy Spirit ( Serm. 75.5) and the benefits of the Holy Spirit ( Serm. 76.4), underscoring the fact that the Holy Spirit differs in no way from the Father and the Son ( Serm. 76.2).
Serms. 78-81 emphasize the fast. Leo urges his flock to recognize the value of fasting, which must be done with love ( Serm. 79.3) and with prompt faith ( Serm. 79.4). As with the other fasts, he urges people to "withhold some small portions of food, so that what is not spent on our tables might benefit alms" ( Serm. 80.1).
THE HEARTS of all Catholics know well, dearly beloved, that today's solemnity ought to be honored among the special feasts. No one doubts how much reverence is owed to this day which the Holy Spirit has consecrated by the wonderful miracle of his own gift. For, from that day on which the Lord ascended over all the heights of heaven to sit at the right hand of God the Father, this day is the tenth. It is likewise the fiftieth from the Resurrection of that same Lord who enlightened us about him from whom light began. It contains____________________