The First Friday devotion and the First Saturday devotion are both reparatory devotions in the Catholic Church that arose from separate private revelations. The First Friday devotion is associated with the Sacred Heart of Jesus, while the First Saturday devotion is associated with the Immaculate Heart of Mary. Incidentally, these are two of the most widely popular devotions to Jesus and Mary.
Both devotions involve receiving Holy Communion on a certain number of consecutive first Fridays or first Saturdays of the month. You can think of it almost like a novena which is prayed on consecutive days for a particular intention, but instead, these devotions are performed on consecutive months for the particular intention of reparation to the Sacred Heart of Jesus and the Immaculate Heart of Mary, respectively.
The devotions are similar in that they call on the faithful to "stand in the gap" by doing penance for sinners, and both give a special promise of Divine assistance at the hour of death, that is, the promise of dying in a state of grace and therefore obtaining heaven.