Today's feast is not a mistaken duplication of February 2! Originally (543) it was the Dedication of New S Mary's By The Temple In Jerusalem; which church in Dom Gueranger's time was thought to be the Al Aqsa Mosque on the Temple Mount (I gather archaeological views about that may have changed).
This is one of the Twelve Great Festivals among Byzantines, from which it reached Saxon England (it was not grudgingly accepted by Rome until 1585), being sometimes named in England "the Offering [Oblatio] of Mary". The mythos that, aged three, Mary was lodged in the Temple as a contemplative (hesychast) and fed by angels with fruit as from the Tree of Life in Eden, highlights her immaculate sanctity and anticipates her bodily and glorious Assumption. The Byzantine texts for this day are well worth dipping into.
In the Liturgia Horarum, following a happy mandate by the Council Fathers in Sacrosanctum Concilium, this feast (sadly only Third Class in 1962 and the Novus Ordo) is dignified with proper Office Hymns. The hymn assigned to the Office of Readings consists of some stanzas from the "Golden Jubilee of the Blessed Virgin Mary", described as "(?) Fourteenth Century". This is the hymn which begins
Salve Mater Misericordiae
mater spei et mater veniae,
mater Dei et mater gratiae,
mater plena sanctae laetitiae,
["Hail Mother of Mercy/ Mother of Hope and forgiveness/ Mother of God and Mother of Grace/ Mother full of holy joy/ (O Maria)"] This 'Jubilee' has a hundred stanzas of which a few are selected nowadays for popular use; a different selection is offered in the Liturgia Horarum.
In 1968, the first draft of the Hymns for the Restored Breviary, produced by Coetus VII under the chairmanship of Dom Anselmo Lentini, had suggested a selection of stanzas from this hymn for the Feast of the Visitation, May 31; Dom Anselmo thought that this hymn was suitable for that day because of the (back in the 1960s) growing popular tradition of celebrating the last day of Marymonth as a celebration of our most blessed Lady as Mediatrix of All Graces. However, this suggestion did not survive into the definitive Liturgia Horarum; instead, a different selection of stanzas from this hymn was assigned to November 21, today.
If you want to enjoy a very cheerful congregational and popular rendering, seek out on Father Ray Blake's blog the full video of the SSPX, celebrant His Excellency Bishop Fellay, offering the Holy Sacrifice in the Concrete Submarine at Lourdes, 27 October 2014 (beginnining of the video). But although the first stanza you will hear, which also functions throughout as a Chorus, is used in the Liturgia Horarum, the other stanzas therein are, as I have said, not those found in the popular selection as in the video.
Lentini, himself a very competent composer of Latin hymnody, described this hymn as "Carmen notissimum ... in suo genere pulcherrimun ac perfectum" ["in its own genre, most beautiful, and perfect ... a very famous poem"].
Surely, a suitable hymn, with a suitably catchy tune, for our Holy Father's Jubilee of Mercy?