Mediator Dei 181-184

Pope Pius XII lays down some cautions:

181. Any inspiration to follow and practice extraordinary exercises of piety must most certainly come from the Father of Lights, from whom every good and perfect gift descends;[Cf. James 1:17] and, of course, the criterion of this will be the effectiveness of these exercises in making the divine cult loved and spread daily ever more widely, and in making the faithful approach the sacraments with more longing desire, and in obtaining for all things holy due respect and honor. If on the contrary, they are an obstacle to principles and norms of divine worship, or if they oppose or hinder them, one must surely conclude that they are not in keeping with prudence and enlightened zeal.

Piety should lead believers to the sacraments.

Some acts of piety are not liturgical as such, and a few are listed and lauded:

182. There are, besides, other exercises of piety which, although not strictly belonging to the sacred liturgy, are, nevertheless, of special import and dignity, and may be considered in a certain way to be an addition to the liturgical cult; they have been approved and praised over and over again by the Apostolic See and by the bishops. Among these are the prayers usually said during the month of May in honor of the Blessed Virgin Mother of God, or during the month of June to the most Sacred Heart of Jesus: also novenas and triduums, stations of the cross and other similar practices.

The fruitful result:

183. These devotions make us partakers in a salutary manner of the liturgical cult, because they urge the faithful to go frequently to the sacrament of penance, to attend Mass and receive communion with devotion, and, as well, encourage them to meditate on the mysteries of our redemption and imitate the example of the saints.

Therefore, these lay developments of the spiritual life cannot be fruitfully stopped, or changed wholescale:

184. Hence, he would do something very wrong and dangerous who would dare to take on himself to reform all these exercises of piety and reduce them completely to the methods and norms of liturgical rites. However, it is necessary that the spirit of the sacred liturgy and its directives should exercise such a salutary influence on them that nothing improper be introduced nor anything unworthy of the dignity of the house of God or detrimental to the sacred functions or opposed to solid piety.

Mediator Dei on the Vatican web site.