By tradition, May is the month dedicated to the Blessed Virgin Mary - Theotokas. The month of May is the "month which the piety of the faithful has especially dedicated to Our Blessed Lady," and it is the occasion for a "moving tribute of faith and love which Catholics in every part of the world [pay] to the Queen of Heaven. During this month Christians, both in church and in the privacy of the home, offer up to Mary from their hearts an especially fervent and loving homage of prayer and veneration. In this month, too, the benefits of God's mercy come down to us from her throne in greater abundance" (Paul VI: Encyclical on the Month of May, no. 1).
Read more about the devotion from Catholic Culture here including traditional prayers, litanies and devotions.