86 In this striving you will be most wonderfully helped by a practice commended by Our Predecessor of holy memory Pius X. This commendation is contained in that "Exhortation to the Catholic Clergy," which he wrote with such unction and affection. This We warmly recommend you to read. In it, among all the means to preserve and increase the grace of the priesthood, he placed first the use of the Spiritual Exercises. This means We Ourselves have also frequently recommended; and particularly in Our Encyclical Letter Mens Nostra, We have paternally and solemnly urged it upon all Our sons, but more especially upon Our Priests. As the year of Our priestly Jubilee drew to a close, We could find no better and more salutary reminder of that happy anniversary, than to give to Our sons an invitation, through the above-mentioned letter, to draw more copiously from the waters of life springing up into life everlasting, this inexhaustible fountain providentially opened by God to His Church. Again now, to you, Our Dear Brethren, who are all the closer to us because you work more directly with Us to establish the kingdom of Christ upon earth, We believe We cannot give better proof of Our Fatherly affection than by exhorting you most fervently to make use of this means of sanctification, to the best of your abilities. Take for your guide those principles and norms laid down by Us in the above-mentioned Encyclical. It is not enough to withdraw to the sacred seclusion of the Spiritual Exercises only at the intervals and in the exact measure prescribed by ecclesiastical law but you should enter into retreat more often and for longer periods, as far as possible to you, and you should consecrate, in addition, a day of each month to more fervent prayer and greater recollection, according to the practice of priests of great zeal.
Source: Ad Catholici Sacerdotii (Vatican.va)