I think much of the answer to that question lies in some of Jesus' last words while hanging on that cross, before finally giving up his life. "It is finished."
So what exactly was accomplished on the cross that first "Good Friday?" Two things, according to the writers of the Bible. God's justice was satisfied, and God's grace and love were demonstrated in a dramatic way, and both of these things were amazingly accomplished in the same act.
See, according to the Bible, God had this plan from the very beginning, before he even created us. He knew we would turn away from him. He knew we would make mistakes. He knew what kinds of tragedies and monstrosities we would create in this world, how we would hurt each other, how we would hurt ourselves. And he knew what it would take to redeem us. Before Jesus came, the Israelites sacrificed animals in the temple, where God's spirit lived, to atone for their sins. But this was not complete atonement. The "wages of [our] sin is death" (Romans 6:23). Only someone without any sin could truly take that punishment for us, for the entire human race. And God knew that he himself was that someone. That's where Jesus comes in. According to the Bible, God came down to earth as a human like us in the person of Jesus, and lived his life as our substitute, perfectly obeying his law, and dying in our place on the cross. He suffered all of God's wrath physically and spiritually so that we wouldn't have to. The Bible says that Jesus was the "propitiation" for our sins, meaning that God did not merely hold back his wrath from us, but rather he exhausted his wrath against sin by pouring it all out on Jesus, on himself, and thus satisfied his justice. God's plan to punish himself for our sin was effectively "finished" on the cross.
At the same time, God demonstrated his incredible grace and love for us by allowing himself to be slaughtered on the cross. Matthew records that at the moment of Jesus' death, "the curtain of the temple was torn in two from top to bottom." The curtain he writes about was the curtain which separated the outer parts of the temple, where the people and the Jewish priests were allowed, from the innermost part, the Holy of Holies, where God's spirit dwelt. There, only the chief priest could go once a year to ask forgiveness for Israel's sins. This, in other words, was the curtain which symbolically separated man from God, from God's presence. This was the curtain which tore in two when Jesus died on the cross. Jesus bridged the gap between us and God by taking our sin upon himself. The separation between man and God was destroyed. Jesus died so that we might not only be released from our deserved punishment, but also enter into God's family, into his house, into a personal relationship with him. God's grace and love were demonstrated at the same time as his justice was satisfied. His plan to not only release us from our bondage to sin, but to also adopt us as his sons and daughters, was "finished" on the cross too. And we all know what happens next.
See, the "good" part about "Good Friday" is the hope that it gives us for Easter Sunday. Good Friday wasn't the end of the story. Jesus came back from the dead. He conquered death once and for all--once and for all for us. Matthew also records that when Jesus died "the earth shook and the rocks split. The tombs broke open and the bodies of many holy people who had died were raised to life. They came out of the tombs, and after Jesus' resurrection they went into the holy city and appeared to many people."
Jesus came to bring us new life. He came to rescue--to rescue us from ourselves. And all he asks is for our trust--that instead of trusting in our own "goodness" or "morality" or that our good deeds will outweigh our bad on the scales one day, that instead of all of that we would trust him for doing what we can't do. Without Good Friday, there would have been no Easter Sunday. And without either of them, there would be no chance for us to escape the consequences of our crimes, and to truly know God in a real and intimate way.
Happy Good Friday everyone.