I am asked where liturgical formulae can be found for our lady Meditrix of All Graces. Happily, for Anglicans who are prepared to turn to the good old English Missal, you can find her in the Appendix of Masses Proper to England and Wales. This is because in Durham, Northumberland, the West Riding of Yorkshire, Herefordshire, and all Wales, this feast was on the Calendar of the corresponding RC dioceses (is it still on the Novus Ordo Calendars of those dioceses?). The date is May 31, except that in later editions of the English Missal, dating from after the institution in 1955 by Pius XII of the feast of the BVM, Queen, on May 31, the older feast of our Lady, Mediatrix, had to be shuffled onto June 1. The Mass can, of course, be said as votive any day when votives are permitted.
Happily, one of the Office Hymns of this beautiful Feast appears in the Liturgia Horarum. In the Common of the BVM, it is the hymn at First Evensong; and it may be used at the Office of Readings as an alternative to Quem terra pontus aethera. Its first line is "Maria quae mortalium". (Sadly, for those who say the Office in English, it is not available; one of the many hymns which ICEL decided not to bother to commission English translations of.) Its author is unknown; it first appears in nineteenth century breviaries.
The Mass was authorised in 1921 by Pope Benedict XV, and among the countries for which it was authorised was, I believe, Belgium. Does it still survive on the Novus Ordo Calendars of such countries?
Can I take this opportunity of pointing out that in the pre-Conciliar days, the Office Hymn at Vespers on feasts of our Lady was always (Rubricarius may correct me) Ave Maris Stella. This is even true of the Immaculate Conception and Our Lady, Mediatrix. As the evening star appeared beside the setting sun, that was the hymn with which Latin Christians addressed her. It may have been Pius XII who infringed this convention by giving a different hymn for the First Evensong of the Assumption; but even he did not tamper with the Second Vespers to the extent of changing the hymn. That step was left for the clever-clogs of the Bugnini school.