The month of May (Overview - Calendar) 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).
This Christian custom of dedicating the month of May to the Blessed Virgin arose at the end of the 13th century. In this way, the Church was able to Christianize the secular feasts which were wont to take place at that time. In the 16th century, books appeared and fostered this devotion.
The practice became especially popular among the members of the Jesuit Order — by 1700 it took hold among their students at the Roman College and a bit later it was publicly practiced in the Gesu Church in Rome. From there it spread to the whole Church.
The practice was granted a partial indulgence by Pius VII in 1815 and a plenary indulgence by Pius IX in 1859. With the complete revision of indulgences in 1966 and the decreased emphasis on specific indulgences, it no longer carries an indulgence; however it certainly falls within the category of the First General Grant of Indulgences. (A partial indulgence is granted to the faithful who, in the performance of their duties and in bearing the trials of life, raise their mind with humble confidence to God, adding — even if only mentally — some pious invocation.
Excerpted from Enchiridion of Indulgences.