According to the Diary of St. Maria Faustina Kowalska, Jesus Christ made a special promise, which she was to communicate to the whole world:
My daughter, tell the whole world about My inconceivable mercy. I desire that the Feast of Mercy be a refuge and shelter for all souls, and especially for poor sinners. I pour out a whole ocean of graces upon those souls who approach the fount of My mercy(Diary, 699).
In three places in her Diary, St. Faustina records a promise from our Lord of specific, extraordinary graces He will make available through the devout reception of Holy Communion on this Feast Day; truly a "whole ocean of graces" is contained in these promises:
I want to grant a complete pardon to the souls that will go to Confession and receive Holy Communion on the Feast of My mercy (1109).
Whoever approaches the Fount of Life on this day will be granted complete forgiveness of sins and punishment (300).
The soul that will go to Confession and receive Holy Communion will obtain complete forgiveness of sins and punishment (699).