The date is not his day of death, which is often chosen for saints, but the anniversary of his priestly ordination
“Pope Francis, assenting to the petitions and desires of the People of God, has decreed that the celebration of Pope Saint Paul VI, should be inserted into the Roman Calendar on 29 May with the rank of optional memorial,” reads a declaration from the Vatican’s liturgy congregation.
The date thus recognizes the anniversary of priestly ordination of this pontiff (1897-1978), who was elected as pope in 1963.
Francis canonized him last October.
Paul VI is most known for bringing the Second Vatican Council to its completion and for his landmark encyclical on artificial contraception, Humane Vitae, published in 1968.
“He exercised his Supreme Magisterium favouring peace, promoting the progress of peoples and the inculturation of the faith, as well as the liturgical reform, approving Rites and prayers at once in line with tradition and with adaptation for a new age. By his authority he promulgated the Calendar, the Missal, the Liturgy of the Hours, the Pontifical and nearly all of the Ritual for the Roman Rite with the purpose of promoting the active participation of the faithful in the Liturgy,” the declaration notes.
It also says:
Paul VI shines out as one who united in himself the pure faith of Saint Peter and the missionary zeal of Saint Paul. His consciousness of being the Successor of Peter is evident when we recall that on 10 June 1969, during a visit to the World Council of Churches in Geneva, he introduced himself by saying “My name is Peter.” … The Church was always, indeed his constant love, his principal concern, the object of constant reflection, the first and most fundamental thread of his whole pontificate. He wished nothing other than the Church would have a greater knowledge of herself in order to be ever more effective in proclaiming the Gospel.