Why "Good" Friday?

So why is the day of Jesus' crucifixion called “Good Friday?" 

Well, no one really knows! In various countries Good Friday is also called Holy Friday, Black Friday, Great Friday, Long Friday, and Silent Friday.
One reason for the name may be that although what Jesus suffered in crucifixion is horrifying, it resulted in our sins being forgiven and the restoration of our relationship with God. That is indeed good news for Creation.

When Jesus said, “It is finished,” he bowed his head and gave up his spirit, (John 19:30).

These are Jesus' final words from the cross and part of "the seven last words of Christ."
 However it is interesting to note that in the Greek, "it is finished" is actually only one word, “Tetelesthai”!  
In Colossions 2:14 St.Paul explains that “the written charge against us is cancelled, nailed to the Cross,” every sin ever committed is forgiven.
 In Roman times "tetelesthai" meant ”the debt is paid in full,” it only remains for each of us to say “yes” to the offer.