A good question, and one to which a surprisingly large number of people get the wrong answer. Some believe that it was the greatest tragedy ever, and to a point it was. Jesus was wrongly tried and convicted of crimes of which He was not guilty. For them, it should be called Bad Friday.
But those who think of it in that way have never understood the purpose of the Cross in the first place. And they certainly have never gloried in the Cross, as Paul and Isaac Watts each say they did.
To others it is just another festival in the so-called Christian callender. It is linked to Easter, or to use the biblical term, Passover. There are church services and we go to the Good Friday service because the church says that that is what we are to do. It is good to attend Good Friday services, but if that it all there is to them, then it differs nothing from other festivals.
To the true Christian, Good Friday is the most wonderful time of the year, and it is called this because God sent His Son into the world to die for the sins of the world, doing that without which no one could ever be saved. Here God "laid on Him the iniquity of us all," (Isa.53:6). It was on Calvary that "the Lamb of God took away the sins of the world," (Jn.1:29). At Calvary, God's Son bore our transgressions and their just punishment so that all who put their trust in Christ are forgiven and saved.
This was, therefore, the best day in the history of the human race. Here Christ was able to say, "It is finished!" The work the Father had given Him to do was completed, with nothing more being needed to be done on God's part. "It is finished!" Your sin and mine was borne by the Redeemer of the world (Calvin's phrase) and through faith in Him we can go free.
Further, it was at Calvary that Christ completely disarmed the powers of darkness and slavery that held the world in captivity. In the Cross, He triumphed over them. "In the Cross of Christ I glory, towering o'er the wrecks of time." On the third day He rose from the dead, thus demonstrating that even death, man's greatest enemy, has been disarmed and destroyed. He is Victor par excellence.
On the Cross the Law giver and Law keeper was put to death as a Law breaker so that Law breakers might become Law keepers. This is why the Cross is so glorious. He took our place and died our death so that we might never taste death. In fact, as Heb.2:9 so eloquently tells us, He tasted death for every man. He died for all, says Paul. Christ is the world's Redeemer, as the hymn-writer affirms.
What happened on Good Friday? The greatest thing the world ever saw. For there, God dealt with His own holiness while at the same time dealt with mankind's sin; and He did these while remaining totally just Himself, and became the justifier of all who trust in Jesus.
Have you trusted in Jesus Christ, God's Son? Have you personally come before His Cross as a broken sinner, full of guilt and shame, and acknowledged your lostness before Him? Do you see yourself as a condemned sinner who has no rights whatever before the great Judge of all the earth?
If you haven't, come to Him now in deep penitence for your sin and sins, seek His forgiveness and pardon, cast yourself unreservedly on His mercy, and accept His salvation. And you know what? The moment you do that, you become a child of God, a sinner saved by grace, and an inheritor of the kingdom of God.