Your comment was quite interesting. I found this:
Pope John Paul II made the surprise announcement of this change in his homily at the canonization of Sr. Faustina on April 30, 2000. There, he declared: "It is important then that we accept the whole message that comes to us from the word of God on this Second Sunday of Easter, which from now on throughout the Church, will be called 'Divine Mercy Sunday.' "
Entire explanation:
http://www.divinemercysunday.com/mercy_sunday.htm
Thanks for the post and the link.
You were right on, Petronski.