Calling the day of the Crucifixion ‘Good’ Friday is a designation that is peculiar to the English language. In German, for example, it 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.
I’ve read that the word good used to have a secondary meaning of holy, but I can’t trace that back in my etymological dictionary. There are a number of cases in set phrases where the words God and good got switched around because of their similarity. One case was the phrase God be with you, which today is just good-bye. So perhaps Good Friday was originally God’s Friday. But I think we call it Good Friday because, in pious retrospect, all that tragedy brought about the greatest good there could be.
I can see virtue in either terminology. If we call it Mourning Friday, as in German, we are facing reality head on, taking up the cross if you will, fully conscious that the Christian walk is seldom a walk in the park. But if we call it Good Friday, as in English, we are confessing the Christian hope that no tragedy—not even death—can overwhelm God’s providence, love, and grace. Either way seems fine to me!
You can read more about this topic if you are wondering if the Crucifixion took place on a Friday.