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The simple answer to the question is Clement VIII did in 1604 as his missal contains differences, albeit only minor ones, to the Ordo Missae and Canon, along with new feasts etc (or rather mainly the re-introduction of feasts suppresseed in the 1570 calendar.

Not sure it helps to maintain there were no changes when clearly there were many over history, particularly from the mid-1950s onwards.

Take today for example.  In the 1570 Missal today is the feast of St. Giles, of simple rite, with a commemoration of the Twelve Brothers and third collect A cunctis.  Following the rubrics, the celebrant of a private Mass may add additional prayers from those given in the Missal as long as the total number is five or seven.

In MR1962 today is a IV class ferial day with the Mass of the preceding Sunday with commemorations of St. Giles and the Twelve Brothers.  

If Quo primum set MR1570 in stone then how come there are things such as IV class, III class, II class etc days when St Pius V's Missal has doubles, semi-doubles and simples?

If Quo primum set MR1570 in stone then how come there are things such as IV class, III class, II class etc days when St Pius V's Missal has doubles, semi-doubles and simples?

The purpose of QP was to codify the liturgy of the Mass; the calendar of feasts has always been consistently updated throughout history.  And such does not affect the liturgy.


Example:  During the Middle Ages, you had all sorts of added customs for the major religious orders.  You have the Dominican rite, the Benedictine rite (these still exist today), but also other rites (or even other dioceses) started to add certain prayers and "extras" to the mass.  The intention was good; the effects were chaotic.  After a few hundred years, there existed a lack of uniformity in the liturgy of the Mass.  A mass said in N.Spain was different from one said in S.Spain.  Not substantially different, but different enough that if the same priest used both liturgies, there might be 20 min difference or a higher-than necessary learning curve.  It was destroying the "universality" of the Church.

So St Pius V recognized the problem and rectified it.  He allowed all rites that had existed over 200 years in 1570 to remain, or to choose the "new/revised" rite.  Many choose the Pius V rite, in a spirit of helping the Church to re-gain unity.  Some rites, like the Benedictine/Dominican, remained, and since those are private religious groups, and since their changes were very minor, these rites do not disrupt the unity of the Mass.

But things like the calendar and feasts of the Church...such have always been updated (and need to be) over the decades.

Ok, my bad.  I was confused by the poster who said that QP was only for the Latin rite and not Eastern Churches.  This isn't true.  But...by and large, most Eastern Churches use rites that are VERY old and weren't affected by QP.  So, it's true to say that QP didn't affect (most) Eastern rites, but it's not true to say that it (theoretically) didn't apply to them.