October 5th, was instituted as the feast of St. Faustina Kowalska on April 30th, 2000 by St. John Paul II. Only 20 years ago this Polish nun to who we attribute the devotion to The Divine Mercy, officially became a Saint.
St. Faustina had mystical encounters with Our Lord at the Blessed Sacrament. The Lord asked to record these revelations in a notebook. These entries were compiled as the Diary of Saint Maria Faustina Kowalska - Divine Mercy in My Soul
The Lord appeared to her as the King of Divine Mercy, the following is an excerpt from her diary describing her vision.
"In the evening, when I was in my cell, I became aware of the Lord Jesus clothed in a white garment. One hand was raised in blessing, the other was touching the garment at the breast. From the opening of the garment at the breast, there came forth two large rays, one red and the other pale. In silence, I gazed intently at the Lord; my soul was overwhelmed with fear, but also with great joy. After a while Jesus said to me, 'paint an image according to the pattern you see, with the inscription: Jesus, I trust in You.'"