The Christian message, the Christian story, goes something like this: God came to earth and we killed him.

God came to earth in the God-man, Jesus, and we nailed him to a cross.
Jesus went around announcing he was God, the Great I AM, who can forgive sins, who was there in the beginning, who gave the law, who had glory with the Father from before creation.
When he was challenged to prove his deity, he did not point to his miracles. After all, many other prophets did signs and wonders.
He said “If you have seen me, you have seen the Father” (John 12:45; 14:9-10). But these statements about revealing the Father are sandwiched around his act of washing the disciples feet (John 13), which of course, was a symbolic, enacted parable of his death. In other words, Jesus said, “You want me to prove to you that I am God? Watch the way I die.” This is not exactly what anyone was expecting -- a God who would express his Godhood in dying. But when we see his death as an act of sacrificial, self-giving love, it starts to make sense. The cross is ugly but glorious.
The gospel is full of paradoxes. His death reveals divine glory. But his death also reveals the utter depravity of man. God shows up on our planet and what do we do to him? We torture and kill him. The light of the world comes and we seek to extinguish it. The bread of life arrives and we seek to tear it apart. The Word of God comes into history, and we want that Word silenced.
The cross simultaneously reveals the saving love of God and the utter wickedness of man.
But it also reveals that God uses human sin to bring about his plan of salvation from sin.
The cross is the ugliest and most beautiful event in history at the same time. It reveals the heart of God and the heart of man at the same time. It displays divine grace and love in the same moment it reveals human sin and wickedness.
Thus, Good Friday really is a good day. It is a day of happy sadness. There is sadness because it is your sin and my sin that put him on the cross. We drove the nails. On this day, we have to reckon with who we are and the evil we have done. This day reveals our depravity.
But there’s also a goodness and happiness that underlies it all.The happiness comes from knowing the cross is our God coming to save us. Even as we sin against him he is saving us from our sin. Our ultimate act of rejecting God accomplished ultimate reconciliation between God and man. Jesus loved us to the end (John 13:1) — which means to the end of his life. It was not actually the nails that kept him on the tree, it was his love for each of us. He was determined to love us to hell and back. God came into this world to enter into our plight, to take our filth and wretchedness upon himself. While he never sinned, he became the representative of sinners. He took the punishment we deserve. He took on the worst that evil could throw at him and defeated it. 
So, yes Good Friday has always been a day of grief for Christians. But out grief is hemmed in by joy that comes from knowing all that Christ did, he did for us. All he went through on Good Friday was to accomplish our salvation. He died that we might live. He was condemned that our guilt might be erased. He was shamed that we might share in his glory. He was stripped naked that we might be clothed in righteousness. He served that we might reign. He died that we might live.
One last thought: Jesus said “If you have seen me, you have seen the Father.” Then he stretched out his arms and died. The cross reveals what God is like. If anyone else made the claim, “If you have seen me, you have seen God,” we would think he was crazy. And we’d be right. But then Jesus goes on to say that the world we will know we are his disciples by the way we love one another (John 13:35), and the same love the Father and Son have for one another should be seen in his disciples’ love for one another (John 17:20ff). In other words, in a very real sense, the church ought to be able to say, "If you have seen us, you have seen (in a very dim way) God. If you have seen us, you have seen the Father, and the Son, and the love they share.” This is the challenge of Holy Week: we are called to love with a cruciform, Jesus-like love. The love he has poured into us is to flow out of us to others.
Much of this post was inspired by the work of Glen Scrivener.