You may have heard that today is called Good Friday because it was on this day that Christ accomplished our redemption and, as Martha Stewart might say, "That’s a Good Thing."
Actually, as intuitive as this answer is, the answer is more complex than that. You will find some dictionaries (like this one) that list the origin of the "Good" in Good Friday as the ordinary adjective good, being taken in the sense of "holy." You will find others (like this one) that disagree.
The Catholic Encyclopedia entry on Good Friday lists its designation in several languages, and although it is called "Holy Friday" in the Romance languages, English isn’t a Romance language but a Germanic one. The article concludes that the origin isn’t clear, but notes that "Some say it is from ‘God’s Friday’ (Gottes Freitag)."
This actually sells this explanation a little short. As far as I can determine, the "God’s Friday" explanation is the standard one, particularly among older etymologists. It’s also reasonable since we know of a similar very common "God" > "good" transformation in English, namely "goodbye," which is a contraction of "God be with you."
Once the true origin of a word or phrase is forgotten, people have a tendency to analyze it in terms of the words it sounds like, and so people today tend to analyze "goodbye" in terms of wishing good for someone, though this isn’t at all where the word comes from. I suspect the same thing is going on with "Good Friday." People are reanalyzing the word "Good" based on the familiar adjective today, and this conjecture has crept into some dictionaries. The older, messier "God’s Friday" explanation strikes me as more likely the correct one.