But what about this horrific death makes it even a mediocre Friday, let alone “Good Friday”? As we Christians believe, it was the day that our sins were atoned for. The guilt and shame of the whole world were on that day piled upon Jesus Christ, and (in the words of this hymn) the wrath of God was satisfied.
How does that work? Wrath -- doesn't that seem kind of, um, judgmental and archaic and so on? The short answer is "no," but let me unpack that a bit.
One of life's big questions is this: Does the universe, the earth, my life mean anything? Or is life, the universe and everything just a meaningless cosmic accident? I'm going to say that the only possible answer is that these things, this planet, your life all mean something. And in your life, your daily thoughts and activities, you also say that life means something. Do you ever get angry at the injustice in the world? When you read about greedy bankers on Wall Street, cruelty and oppression in Syria, the horror of human trafficking? You're angry because you believe these things should not be! They're wrong!
Where do the moral facts of the universe come from? They're not based on your opinion or mine; contrary to the non-cognitivist view of meta-ethics, moral statements are not in the same category as "Go Bears!"; they are rather more like "It's evil to kidnap children and force them to become soldiers or prostitutes because it robs them of dignity and crushes their spirits." And it's not reasonable to say "well, child prostitution may be evil to you, but it's okay by me."
Right? So where do such moral facts come from?
Christians believe that they come from the Creator of the heavens and the earth. We call Moses the Lawgiver, but he was the law conduit; the moral facts of the universe came from God the Creator.
Some of these moral facts are well-understood by most of us -- the obligation to treat each person fairly (Lewis wrote cogently about this in Mere Christianity) for example. "Hey, what did you do that for? What did I ever do to you?"
One of the facts which has become unpopular over the centuries is the need for atonement when we do wrong. Lewis gave a terrific allegory of this in the closing chapters of The Lion, the Witch and the Wardrobe, but the short version is that in the words of the Bible, "the wages of sin is death but the free gift of God is eternal life through Jesus Christ our Lord" (Romans 6:23).
Anyway, what all that means is: Christ lived a sinless life and his death paid the penalty for our sins (1 Peter 2:21-24). This was a wonderful day for you and me, even if you aren't a Christian (1 John 2:1-3), but a horrific one for Jesus, who gave his life for us.
And that's why we call it Good Friday.