"For the sake of his sorrowful passion, have mercy on us" is hardly fitting for the paschal season.
Compared with the resurrection hymns of the Byzantine rite:
"Christ is risen from the dead, by death trampling death, and to those in the graves granting life."
"By death, by death you conquered death."
We aren't supposed to be commemorating Christ's wounds during Pascha, but rather, his victory over death.
I also have a serious problem with basing a feast of the church, even if it is only a feast of the Latin Church, on a private "revelation."
I personally do not believe Faustina's "revelation" was anything of the kind.
The Feast of Mercy (Divine Mercy Sunday)
Jesus said to St. Faustina:
I desire that there be a Feast of Mercy. I want this image, which you will paint with a brush, to be
solemnly blessed on the first Sunday after Easter; that Sunday is to be the Feast of Mercy. (Diary, 49)
I desire that this image be displayed in public on the first Sunday after Easter. That Sunday is the
Feast of Mercy. Through the Word Incarnate I make known the bottomless depth of My mercy. (Diary, 88)
I desire that the first Sunday after Easter be the Feast of Mercy. (Diary, 299)
And who knows anything about this feast? No one! Even those who should be proclaiming My mercy and
teaching people about it often do not know about it themselves. That is why I want the image to be
solemnly blessed on the first Sunday after Easter, and I want it to be venerated publicly so that every soul may know about it. (Diary, 341)
No soul will be justified until it turns with confidence to My mercy, and this is why the first Sunday
after Easter is to be the Feast of Mercy. On that day, priest are to tell everyone about My great and
unfathomable mercy. (Diary, 570)
Yes, The first Sunday after Easter is the Feast of Mercy, but there must also be acts of mercy, and I demand the worship of My mercy through the solemn celebration of the Feast and through the veneration of the image which is painted. By means of this image I shall grant many graces to souls. (Diary, 742)
copied from The Foundation of the Divine Mercy Sanctuary ("... Rev. Ignacy Rozycki, STD - his research was part of the official investigation into St. Faustina's life...", copied from Understanding Divine Mercy Sunday, pdf file - page 19)
[ http://www.sanktuarium.krakow.pl/english/04c.html ]
[ http://www.marian.org/divinemercy/understanddms.html ]
I desire that the first Sunday after Easter be the Feast of Mercy (Diary 299) - the Lord Jesus said to Sr. Faustina. In the following years, Jesus returned to this demand in as many as 14 different apparitions, establishing very clearly the location of this Feast in the Church's liturgical calendar, and defining the reason and objective of its institution, the manner in which preparations for it and the celebration itself were to be conducted.
( http://www.sanktuarium.krakow.pl/english/04c.html )