What about Good Friday is good if it marks the day Jesus died? It is easy enough to say that Jesus' death was "good" in that it saved mankind. Yet, somehow so brief an explanation as that just doesn't seem to do the crucifixion justice as an event that changed history. The words seem pitifully inadequate.
This isn't an uncommon occurrence when one tries to use something as mundane as language to describe the workings of God. Consider something as basic and essential as the doctrine of the Trinity...most all Christians could give you a rudimentary explanation of the Trinity: "It means that God is one being in 3 persons." While that explanation is accurate, it raises more questions than it answers.
This disconnect is just the logical threads of human experience failing to wrap around the divine perspective of God. To man, 1 isn't 3, death isn't good, and celebrations are not unhappy. That is what this day is however. Today is an unhappy celebration of the radical, life-altering, time-changing truth of Christ's sacrifice. Today marks the day that the God-man Jesus Christ suffered and died so as to bring eternal hope and joy.