Er, I'm not a hundred percent sure, but from what I remember from catholic school (which isn't much) the reason it's "Good" Friday is because by being crucified, Jesus essentially dies for our sins, and thus it is possible for us to enter heaven (yay), which even to this day I'm not sure how that whole process works.

Supposedly, before Christ's death , we couldn't enter heaven because Adam and Eve fucked everything up by sinning (eating fruit from the tree of knowledge). Thus from that point on, all of mankind is born with original sin, so God decides to send his only SON down to earth for the sole purpose of being KILLED so that OUR sins (eating fruit) can be forgiven...

...please keep in mind that this interpretation has been colorized by fervent skeptic.

but ya, it's "Good" because now we don't have to wallow in the hell that is (fruity) knowledge. We can enter heaven simply by believing. It's logical and Illogical all at the same time.