Why the Holy Sacrifice of the Mass is Offered at the Hour of Terce

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Dom Gueranger's The Liturgical Year, Monday within the Octave of Corpus Christi, discussing the work of the Holy Ghost in the Holy Sacrifice of the Mass (Volume X, pg. 351):

"'The priest,' says St. John Chrysostom, 'comes forth, carrying, not fire, as under the Law, but the Holy Ghost.' It is a man who appears before us, but it is God who works.' 'How shall this be done?' said Mary to the angel, 'for I know not man.' Gabriel answers her: 'The Holy Ghost shall come upon thee, and the power of the Most High shall overshadow thee.' 'And thou now askest me,' says St. John Damascene to an inquirer, 'how do the bread and wine and water become the Body and Blood of Christ?' I answer thee: 'The Holy Ghost overshadows the Church, and achieves this mystery, which is beyond all word and all imagination.'

Therefore it is that, as St. Fulgentius observes, the Church could not more seasonably pray for the coming of the Holy Ghost, than at the consecration of the Sacrifice, wherein, as under the shadow of the Spirit in the Virgin's womb the Wisdom of the Father united Himself with the Man chosen by Him for the divine espousal, so the Church herself is united by the Holy Ghost to Christ, as a bride is to her spouse, or the body to its head. It is on account of this that the hour of Terce (nine o'clock), the hour wherein the divine Paraclete came into this world (at Pentecost), is the one set apart by the Church, on each of her festivals, for the solemn celebration of the great Sacrifice, over which this blessed Spirit presides in the omnipotence of His operation."

Adapted from here.