Good Friday is a day that proclaims God's purpose of loving and redeeming the world through the cross of our Lord, Jesus Christ. It is a day that is good because God was drawing the world to God's self in Christ.
As seen in John's gospel, particularly, God was in control. God was not making the best of a bad situation, but was working out God's intention for the world — winning salvation for all people.
We call it "good" because we look backward at the crucifixion through the lens of Easter!