Many traditionalist Catholics, particularly those devoted to the Traditional Latin Mass (TLM), are uneasy with the Divine Mercy devotion which originated from the private revelations of St. Faustina Kowalska in Poland in the 1930's. On the other hand, Divine Mercy Devotion has overwhelmingly increased since Pope St. John Paul II promoted it regularly during his pontificate. When John Paul II finally canonized Sr. Faustina on April 30, 2000, he established a new feast day, renaming the Second Sunday of Easter to Divine Mercy Sunday.
There are many arguments pro and con for this relatively new devotion. If you'd like to read a little more about the controversy, before we actually arrive at Divine Mercy Sunday in a few days, allow me to point you to an interesting article by a traditionalist priest, one who celebrates the TLM only. You may be surprised then when you hear him say this: "I very much believe the Divine Mercy apparitions to St. Faustina were not only authentic, but a great gift from heaven to earth in the darkest century of both world history and Church history."
The rest of his article has helpful links and makes for some very good reading!
Happy Feast Day of Divine Mercy to one and all!