Your pertinent question deserves to be told the perfect true reason. After the Ascension of our Saviour which took place on the fortieth day after His Resurrection, the apostles returned from the Mount of Olives, on which He had suffered them to see Him when He was returning to the Father, as the book of the Acts of the Apostles also testifies, and entered Jerusalem and are said to have waited ten days for the coming of the Holy Ghost, and when these were fulfilled on the fiftieth day they received Him with joy. And thus in this way the number of this festival was clearly made up, which as we read was figuratively foreshadowed also in the Old Testament, where when seven weeks were fulfilled the bread of the firstfruits was ordered to be offered by the priests to the Lord: [2193] and this was indeed shown to be offered to the Lord by the preaching of the Apostles which they are said on that day to have addressed to the people; the true bread of the firstfruits, which when produced from the instruction of a new doctrine, consecrated the firstfruits of the Jews as a Christian people to the Lord, five thousand men being filled with the gifts of the food. And therefore these ten days are to be kept with equal solemnity and joy as the previous forty. And the tradition about this festival, transmitted to us by Apostolic men, should be kept with the same uniformity. For therefore on those days they do not bow their knees in prayer, because the bending of the knees is a sign of penitence and mourning. Wherefore also during these days we observe in all things the same solemnities as on Sunday, on which day our predecessors taught that men ought not to fast nor to bow the knee, out of reverence for the Lord's Resurrection.

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Footnotes:

[2193] Cf. Deuteronomy 16:9.