Good Friday is the day where we as Christians remember that Jesus Christ died. And for Christians, this day is the day where Christians remember the pivotal moment of human history where God effected the means of the redemption of humanity. Good Friday celebrates death, because Christianity says that by death of one, the many can be reconciled to God. This is epitomized in a passage from the prophet Isaiah:
All we like sheep have gone astray;
we have all turned to our own way,
and the Lord has laid on him
the iniquity of us all.
This is the concept of atonement. By dying on the cross Jesus extended both down to people and up to God. By dying he went down to defeat the forces of death and darkness; by being a sacrifice he looked up and became for the ransom for our sins and satisfied the wrath of God; and by performing an act of love horizontally became our example. He is the victor over death, the payment for our wrongs, the ransom for our sins, and our greatest human example. And this death was for us; it’s not the Jews or the Romans that killed Christ (though they had their part). It is us, you and I, in rebellion towards God, in all our sin that necessitated the death of God and man.
For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
2 Corinthians 5.21