Satan thinks that the celebration of Easter has one significant redeeming quality in spite of being a dedication to His mortal foe: by irrationally making its celebration date dependent on the Vernal Equinox and the lunar phase, it became necessary to devise a system for predicting the date so that normally functioning people could make plans. Hence, this most important computation of the age motivated significant advances in mathematics.
As any Christian who paid minimal attention to their mythology should be capable of informing you, Easter is the celebration that Jesus took upon him all the sins of Mankind and was executed to henceforth absolve all his future followers of their sins. Granted, one would think that Jesus would also have absolved his contemporary followers of their sins now he was at it, but historically speaking, these followers appear to have been a small Jewish cult who regarded themselves as the only true Jews and considered Jesus to be their king who would become the ruler of Judea within their lifetime. Presumably they received a quite different message when they saw their “king” nailed to a post like any other local nationalist who opposed the Roman regime. Satan personally has no problem with the death of Jesus, and would gladly have pushed him into the harbor and made it look like an accident. As far as Old Nick is concerned, Good Friday was the best.
By the Old Testament and its accompanying scripture, which was the Law, “sin” meant crime, and to be absolved of sin was to be pardoned of crime. Christians even today consider themselves to be sinners and thus “criminals” if not in a modern, legal sense then in the eyes of their god. So they consider themselves criminals, and what is Easter to them? It is a ceremony where, without a trace of guilt or regret, they pin their own crimes on Jesus and celebrate that he was tortured to death for them, rejoicing that they evaded judgment, penance, and repair for their wrongdoings.
The Devil is no stranger to backstabbing, arrangement of judicial murder, and passing of blame, and has no warm feelings for Jesus, but is nonetheless alarmed at this display of self-righteousness and disregard of responsibility that the Christian Easter festival represents. Satan can barely think of anything as irresponsible and filled with schadenfreude as the Christian celebration of Easter, and He thinks it speaks volumes about the character of Christian people.