Actually, the term 'good' is unique to the English language. According to Ken Collins, in Germany this day is called Karfreitag. “The Kar part is an obsolete word, the ancestor of the English word ‘care’ in the sense of cares and woes, and it meant mourning. So in German, it is Mourning Friday. And that is what the disciples did on that day—they mourned. They thought all was lost.”
The origin of the name Good Friday, according to some scholars, comes from an Old English synonym (good) for "holy." Others argue it is a corruption of the word "God," in the same way that "Good Bye" comes from the phrase "God be with ye."
Christians call this day “good” because Easter represents the culmination and the victory of Jesus’ “Good News,” or Gospel. Christ’s death is the fulfillment of God’s judgment on sin. Oswald Chambers puts it beautifully: “There is nothing in time or eternity more absolutely certain and irrefutable than what Jesus Christ accomplished on the Cross— He made it possible for the entire human race to be brought back into a right-standing relationship with God.” That is GOOD NEWS for us all!
Have you also wondered about the three days and three nights? There aren’t three days and three nights between Friday and Easter Sunday, so how is this explained? The scriptural reasoning is that the saying is idiomatic. It occurs elsewhere in scripture. Ken Collins notes that Jonah spent three days and three nights in the belly of the whale. But he was swallowed by the fish one day and spit out on the third day (Jonah 1:17—2:10). In order for there to be three days there could only be two nights separating them. Similarly, in Esther 4:16—5:1, there is a fast for ‘three days and three nights’ that begins on the first day and ends on the third day, which means only two nights were involved.
Jesus was buried on Friday, the day of his death, and rose three days later from the dead, breaking forever the bondage of sin over mankind. Chambers says “The heart of salvation is the Cross of Christ. The reason salvation is so easy to obtain is that it cost God so much. The Cross was the place where God and sinful man merged with a tremendous collision and where the way to life was opened. But all the cost and pain of the collision was absorbed by the heart of God.”
Good News and Happy Easter