Good Friday; the Christian oxymoron.
Talk about your classic conflict in terms. The day Christians believe their most popular figure, Jesus Christ, was crucified is called Good Friday. It's the ultimate oxymoron, like military intelligence or plastic glass. Let's face facts: the stuff commemorated on Good Friday – torture, humiliation and death–would seem to mean Good Friday was only "good" to the guys who killed Christ.
Actually, Good Friday, as the official church name for the Friday before Easter, did not even come into common usage until the fourth century A.D. and then it was specifically described in church books as a "day of mourning, not a day of festive Joy."
Likely, as much as this probably bugs Christian authority figures, the most solid and accurate answer to the question of why it's called Good Friday is that some hard-of-hearing monk accidentally added an "o" to "God's Friday," dropped the "s" and moved on.
In fact, in countries other than America, it's referred to as everything from Great Friday to Long Friday. In Germany, it is called "Friday of Lamentation," which is no fun for people who need to put it on a church marquee. In Armenia, it is "High Friday," making you wonder what they're smoking. American atheists and agnostics, the folks who don't believe in Jesus or aren't sure whether or not they do, simply call it "Friday."
Good Friday is the Friday before Easter, and it's traditionally a time of not eating and begging forgiveness for your sins. For Christians, Good Friday and Easter are the main centerpieces of their religion. It's all about Jesus getting killed, allegedly coming back to life and then taking off for heaven. In other words, the whole basis of the Christian faith.
Bottom line (and it's not a very good bottom line): no one seems to be able to answer the question of how Good Friday got its name with any degree of certainty or proof.