Why do we celebrate the Virgin Mary in May?

The Holy See’s Directory of Popular Piety says the following in No. 191:

… since the month of May largely corresponds with the fifty days of Easter, the pious exercises practiced at this time could emphasize Our Lady’s participation in the Paschal mystery (cf. John 19, 25-27), and the Pentecost event (cf. Acts 1, 14) with which the Church begins: Our Lady journeys with the Church having shared in the novum of the Resurrection, under the guidance of the Holy Spirit. The fifty days are also a time for the celebration of the sacraments of Christian initiation and of the mystagogy. The pious exercises connected with the month of May could easily highlight the earthly role played by the glorified Queen of Heaven, here and now, in the celebration of the Sacraments of Baptism, Confirmation and Holy Eucharist.