A kind benefactor once sent me a 1957 ORDO - beautifully bound - of the Province of San Antonio (my guess is that that is in America Septentrionalis). That ORDO has a number of Roman documents printed at the beginning of it revealing that 1957 is a most significant year. It comes just after the first major footmarks were printed upon the Roman Rite by that towering Punic figure, Hannibal 'Non-sum-delendus' Bugnini. The new Holy Week Order had emerged not long before and was to be observed in accordance with a decree of the SCR of 15 March 1956.
This 'reform' was in fact more radical than the reforms that followed Vatican II; however, the producers of that Holy Week book got away with it because the vast bulk of God's People had for centuries not attended the liturgical Rites of Holy Week; in many places only a lay and clerical elite had done so. And what happens only once a year may anyway not be quite as deeply inscribed within you as what marks your Christian life weekly or daily.
Less well known is the Decree Cum nostra of the SCR (March 23 1955) simplifying the rubrics of the Missal and Breviary. Tucked away in the Decree is a bit of methodology that was to prove the weapon of first choice among the radical liturgists of the mid-twentieth century: these changes were imposed by, but not confected by, the mandarins of the SCR; they were actually devised by a special (peculiaris) Commissio of experts (periti) - which included Annibale nostro.
This was when a scythe cut through all but seven vigils and all but three octaves. Commemorations were not to exceed three. First Vespers were abolished except in the case of first and second class feasts and Sundays. What we now call an 'optional memoria' was invented. Variable Last Gospels were, except at Christmas, abolished.
Of course my list does not include a myriad of details which, so much has our liturgical culture changed, would now require a great deal of exegesis for many readers. The Bugninis of this world are always best at the broad brush. Because periti had devised these 'reforms' (and not the hands-on pedants of SCR whose entire lives had been spent spotting in advance how a minute twitch upon the Calendar here would have a consequence there), there were innumerable unforeseen knock-on effects. Dubia streamed into the offices of the SCR and Responsa had to be issued less than three months later.
There are signs that the mandarins had rightly become suspicious of the slipshod workmanship of the Commissio; this time they asked the views of the Commissio but then carefully themselves went through the matters that had been raised. But that did not prevent a new crop of dubia being thrown up when the attempt was made to put the Decree into effect for a complete liturgical year (Advent 1956-Advent 1957). Perhaps by now the SCR was getting embarrassed at having to cart admissions of shoddy drafting down to the editorial offices of Acta Apostolicae Sedis; the next crop of Responsa was published only in Ephemerides Liturgicae, and the Cardinal Prefect of the SCR apparently didn't bother to sign it or have it sealed.
The period from 1955 until 1967 is a single, coherent, period of slashing and ripping which became ever wilder and ever less respectful of the liturgical inheritance of the Latin Church. People say that it is the first act of embezzlement or adultery that can be difficult; then one soon gets comfortably into the culture of it. Something very similar is true of liturgical 'reform'. The 1955 Decree already includes those sinister words generalis instauratio liturgica. That Decree, and the Missal of 1962, and the Conciliar document Sacrosanctum Concilium, and the Novus Ordo, are all simply episodes in a roller-coaster ride that very quickly got completely out of control and clearly would have done so if no Council had ever been summoned. Even Mgr Lefebvre failed to recognise this until he was already almost in the water at the bottom of the big slope.
Pius XII was the (albeit unconscious) begetter of the Novus Ordo.