3 Because the month of May is a powerful incentive to more frequent and fervent prayers, and because our petitions more readily find access to her compassionate heart during it, it has been a favorite custom of Our predecessors to choose this month, dedicated to Mary, for urging the Christian people to offer up public prayers whenever the needs of the Church demanded it or some grave crisis threatened the human race. This year, Venerable Brothers, We in turn feel compelled to call for such prayers from the whole Catholic world. Looking at the present needs of the Church and the status of world peace, We have sound reasons to believe that the present hour is especially grave and that a plea for concerted prayer on the part of all Christians is a matter of top priority.
Source: Mense Maio (Vatican.va)