The Baltimore Catechism answers this paradox thusly: "We call that day 'good' on which Christ died because by His death He showed His great love for man, and purchased for him every blessing."
In addition to remembering Christ's wondrous love for us, we also have occasion to remember something else "Good" today: the holy cross.
One of my favorite things about Passiontide is being able to hear the Preface of the Holy Cross (from Passion Sunday through Maundy Thursday), which I think sums up perfectly the awesome power of the cross:
It is truly meet and just, right, and for our salvation, that we should at all times and in all places give thanks to Thee, holy Lord, Father almighty, eternal God: Who didst establish the salvation of mankind on the tree of the cross: that whence death rose, thence also life might rise again, and that he who overcame by a tree, by a tree also might be overcome . . . .The ultimate irony, expressed above, is that Satan was conquered by the same thing by which he originally drew mankind away from God -- and on that day, he may have thought that he had won. By all appearances, it looked as if evil had triumphed on that "Good" day. But the Father's plan, ordained from the beginning of time, called for the ancient foe to be vanquished by a tree.
St. Ephrem the Syrian expresses this in one of his homilies on the Crucifixion:
He who was also the carpenter's glorious son set up his cross above death's all consuming jaws, and led the human race into the dwelling place of life. Since a tree had brought about the downfall of mankind, it was upon a tree that mankind crossed over to the realm of life. Bitter was the branch that had once been grafted upon that ancient tree, but sweet the young shoot that has now been grafted in, the shoot in which we are meant to recognize the Lord whom no creature can resist.
We give glory to you, Lord, who raised up your cross to span the jaws of death like a bridge, by which souls might pass from the region of the dead to the land of the living. We give glory to you who put on the body of a single mortal man, and made it the source of immortality for every other mortal man.
Ecce lignum Crucis, in quo salus mundi pependit... Venite, adoremus.