This new compulsory memoria is to be observed on the Monday after Whit Sunday. The fact that the Decree establishing it has emerged from the CDW and not from Ecclesia Dei makes clear that it applies to the Ordinary and not at all to the Extraordinary Form. This is confirmed beyond any doubt by the phraseology of the Decree and the details of the Propers issued. Incidentally, the Decree should have provided (as the law does with regard to the - also movable - memoria of the Immaculate Heart) what happens in years when this movable memoria coincides with an immovable compulsory memoria.
The intelligent thing about this innovation is, of course, that it associates our Lady's Motherhood of the Church with the Day of Pentecost, when she sat in the midst of the Apostles as they received the empowering Spirit. But I hope it will not be misused to draw our blessed Lady into the promotion of Bergoglian ecclesiological errors regarding the alleged role of the Holy Ghost in daily inspiring the Roman Pontiff to espouse or disseminate new doctrine.
More broadly ...
Whit Monday, Monday in the Octave of Pentecost when we celebrate with Paschal joy the Gift of the Spirit, is one of the great days in the Traditional Christian Year. Indeed, it is only comparatively recently that it ceased to be a Public Holiday in my own Country. According to my diary, May 21 is still in this year 2018 a Public Holiday in Austria, Belgium, Canada ('Victoria Day'), Denmark, France, Germany, Hungary, Luxembourg, and Holland; and, in Byzantine Rite countries, Cyprus, Greece, and Romania.
Gradually, followers of the Roman Rite have been edging back to a proper observance of the Octave of Pentecost. I regret any initiative which contradicts this tendency. I regretted, a few years ago, the creation of a Feast of Christ the High Priest on the Thursday of Whit Week (happily, this Feast is not universally obligatory). I regret all initiatives which go directly against the hope of Pope Benedict XVI that the two forms of the Roman Rite would gradually (painlessly and organically) converge.
In the Ordinariates, the Days in the (restored) Pentecost Octave rank higher than Obligatory Memorials in the General Calendar, so, most fortunately, the new memoria will be permanently occluded. I very much hope there will be no tinkering with this laudable provision!!
Pentecost Monday is also treated with respect in the Byzantine Rite.
And in the Extraordinary Form we already have two feasts of our Lady's Maternity. I would set no diriment impediment to the festival on October 11 having 'and Mother of the Church' added to its title!
I sha'n't consider Comments until I've published the three parts of this. Tomorrow, I shall offer a little background about B Paul VI and Vatican II; and, on Saturday, some words about problems offered by the Hymns.