"One of the most important aspects of the Fatima message was Our Lady’s request for the First Saturday devotion. In the July 13, 1917 apparition at Fatima, Our Lady said to the children, “You have seen hell where the souls of poor sinners go. To save them, God wishes to establish in the world devotion to my Immaculate Heart…I shall come to ask for the Communion of Reparation on the first Saturdays.” She came expressly with the request for this devotion when she appeared to Sister Lucia on Dec. 10, 1925, in Pontevedra, Spain, where she was preparing for religious life.

Once you have completed five consecutive First Saturdays, continue to practice the devotion monthly for the sake of obtaining graces for the salvation of other souls."

Taken from:
https://www.bluearmy.com/first-saturday-devotion/

To practice the Five First Saturday devotion, Our Lady told Sr. Lucia:

"I promise to assist at the hour of death, with the graces necesary for salvation, all those who, on the first Saturday of five consecutive months, shall 

• confess
• receive Holy Communion
• recite five decades of the Rosary, and
• keep me company for fifteen minutes while mediating on the fifteen mysteries of the Rosary, with the intention of making reparation to me."