In 1954, Ven. Pope Pius XII solemnly proclaimed the Queenship of the Blessed Virgin Mary, and ordered that a feast should be celebrated in honor of her queenly dignity every year on the 31st of May. This feast would thus crown the month of May, in which she has long been specially honored. Up until that time, a feast of Mary under the title of Mediatrix of All Graces was often celebrated on May 31, the truth of which would now be subsumed by the new feast. On the New Calendar, I think the feast of Mary's queenship now appears sometime in October, having been replaced by the feast of the Visitation on May 31, which used to fall on July 2.
Here are a couple of the most important paragraphs from Ad Caeli Reginam, in case you don't have time to read the whole thing (it's not that long):
47. Since we are convinced, after long and serious reflection, that great good will accrue to the Church if this solidly established truth shines forth more clearly to all, like a luminous lamp raised aloft, by Our Apostolic authority We decree and establish the feast of Mary's Queenship, which is to be celebrated every year in the whole world on the 31st of May. We likewise ordain that on the same day the consecration of the human race to the Immaculate Heart of the Blessed Virgin Mary be renewed, cherishing the hope that through such consecration a new era may begin, joyous in Christian peace and in the triumph of religion.
48. Let all, therefore, try to approach with greater trust the throne of grace and mercy of our Queen and Mother, and beg for strength in adversity, light in darkness, consolation in sorrow; above all let them strive to free themselves from the slavery of sin and offer an unceasing homage, filled with filial loyalty, to their Queenly Mother. Let her churches be thronged by the faithful, her feast-days honored; may the beads of the Rosary be in the hands of all; may Christians gather, in small numbers and large, to sing her praises in churches, in homes, in hospitals, in prisons. May Mary's name be held in highest reverence, a name sweeter than honey and more precious than jewels; may none utter blasphemous words, the sign of a defiled soul, against that name graced with such dignity and revered for its motherly goodness; let no one be so bold as to speak a syllable which lacks the respect due to her name.