I have no idea. I've been wondering this the whole day and Google hasn't really given me any really satisfying answers. The only real answer is that 'Good' is perhaps derived from 'God', just like 'Goodbye' is derived from 'God be with you'. So maybe its God's Friday...
Other reasons are to do with all the good that came of this day. Yet, I still can't really reconcile such a painful day with the word 'good'.
I don't know. Maybe Bilbo's linguistics skills can help me out here :)
Here's a photo of the 3pm Good Friday service that we attended. We were late and had to sit outside. They had it fully enclosed and had a portable air conditioner every few meters so it was cool. I was surprised at how effective their simple cloth enclosure and temporary roof was.