John Mason Neale: A Commentary on the Psalms, Psalm 50/51: Volume II, pp. 159-160

WE have now reached the Psalm of all Psalms; that which of all inspired compositions has, with the one exception of the LORD'S Prayer, been repeated oftenest by the Church. And there are almost as many mysteries in its position as there are in its structure.

As to the way in which the Church has repeated it. Till the last reformation of the Roman Breviary, it was said at every Hour, concluding the service, with the exception of Christmastide and the Great Forty Days. So, for some thirteen hundred years, this Fifty-first Psalm, in thousands of congregations, was repeated seven times daily. As S. Augustine says—and what small cause had he, compared to ourselves, thus to exclaim—-" O most blessed sin of David, So gloriously atoned for! O most happy fault, which has brought in so many straying sheep to the Good Shepherd." And further n0tice the position of this Psalm as the L (Vulgate). The psalm, then, as the year of jubilee: S0 they all, those great lights of the Church, Origen, S. Hilary, S. Ambrose, S. Thomas, Cassiodorus, S. Jerome. Compare with this, as the great saints have done, the law given on the fiftieth day after the people had departed from the land of Egypt; compare the parable of our LORD about the two debtors, the one that owed five hundred pence, and the other fifty ; especially remembering the sin of David, which you will, in this Psalm; compare the penalty of fifty shekels of silver inflicted on him who, among the Israelites, had dishonored a virgin. Think also of the fifty just men with whom Abraham's petition began; think of the width of the Ark, fifty cubits, that ark which was to save all those who were saved from the general ruin; further, of the breadth of the tabernacle which Ezekiel in vision beheld: and lastly of the freedom of Levites from the servile work of the Tabernacle when their fiftieth year had been attained. But above all, the year of jubilee, when all debts were remitted, all manors returned to their original owners, all slaves were liberated, all prisoners were set free-- this above all other interpretations, sets forth to us the mystery of the glorious fifty.