Lex supplicandi legem statuat ... er ...

 Veneranda nobis Domine huius diei festivitas opem conferat sempiternam, in qua sancta dei genetrix mortem subiit temporalem, nec tamen mortis nexibus deprimi potuit, quae filium tuum dominum nostrum de se genuit incarnatum.

'Gregorian'; Leofric; Sarum. The Sarum Secreta (and cfr 'Greg') explains the purpose of the Assumption: 

Grata tibi, quaesumus, Domine, munera nostra efficiat Dei Genetricis oratio, quam etsi pro conditione carnis migrasse cognoscimus, in coelesti gloria pro nobis apud te iugiter orare sentiamus.

She died; she could not be held down by the bonds of Death; in heavenly glory we know that she never ceases to intercede for us.

This used to be expressed in the Roman Secret for the Vigil: "the precise reason why You transferred her from this present aion was so that, with complete confidence (fiducialiter) she might intercede for our sins". (idcirco de praesenti saeculo transtulisti, ut ...). The delicious concision and precision of this formula actually survived Pacelli; it was the vandals of the 1960s that did for it. 

Vatican II (Sacrosanctum Concilium 23) had required that there be NO LITURGICAL CHANGES (ne fiant ...) unless the vera and certa utilitas of the Church demanded it: "exigat" [enforces; compels] ... ... But ...

Goodness me, what an unwholesome gang of unscrupulous crooks those men were ...

But look eastwards: there is Good Stuff in Lossky (1903-1958): "Like her Son, she was raised from the dead and borne up to heaven--the first human hypostasis in whom was fulfilled the final end for which the world was created ... She has crossed the frontier which separates us from the age to come. This is why, freed from the limitations of time, Mary can be the cause of that which is before her; can preside over that which comes after her. She obtains eternal benefits. It is through her that men and angels receive grace. No gift is received in the Church without the assistance of the Mother of God, who is herself the first-fruits of the glorified Church."

S Gregory Palamas (c1296-1359), Archbishop of Thessalonica, calls Mary's Passing a Metabiosis, 'a radical change of life'. We recall our Western term Transitus. Perhaps these are more fruitful words than either Koimesis or Assumptio. She has 'Passed Across' into the Eschaton. For Palamas, "she alone is the Boundary (methorion) of Nature both created and uncreated; and nobody could come to God except through her so as to be verily enlightened".

In Byzantium, August 15 is preceded by a 'Little Lent', and it is then a week before the faithful 'bid farewell' to the Feast; in our Latin West, the Day once had a penitential Vigil and was then honoured by a following Octave. The latter decades of the twentieth century, effectively, put the stoppers on all that sort of business. 

But this humble writer would willingly trade in the flowery Marian rhetoric of Papa Pacelli and his associates for the ancient, venerable, beautiful prayers of the Latin Churches; and for their (theologically significant) millennial liturgical arrangements honouring the Passing Across of Mary.