The First Friday observance is made in response to promises made to a nun, now a saint, whose name I cannot remember (help me here) who reportedly received locutions from the Sacred Heart of Jesus in a 17th C French convent and was told to promote devotion to the love and mercy of Jesus and adoration of His Sacred Heart. There is a long list of promises, and I can’t find the website, but I am sure someone will post it soon.
The historical background is that France and much of Catholic Europe was in the grip of the Jansenist heresy (at the same time Protestants were being dominated by Puritans), which preached a very bitter and hopeless view of salvation, and the possibility of forgiveness and absolution from sin through the sacraments. The devotion was promoted, with the blessing of this nun’s confessor, the bishop, later approved by the Vatican, in order to reawaken awareness of the mercy and forgiveness Jesus gives us. One of the promises made through this nun by the Sacred Heart was that by observing prayer and fasting, reception of the sacraments, and particularly participation in Mass on a series of first Fridays that a plenary indulgence could be obtained that would protect one at the hour of death. I apologize, most of my reference materials are at work (I use the forums at home only).
