Today was Good Friday for Christians, the day on which Jesus Christ was crucified and died. What’s so good about it then?
Rather, the most likely explanation is that it comes from a now obsolete use of good to mean holy. This is probably the case, considering the fact that most other days in the week before Easter (Holy Week) are prefaced by Holy.
Still, it’s a little unusual that we use Good for Good Friday and not the other days, considering that it should be the most depressing day of the year for Christians.
I think using Good was a way for Christians to emphasise the overall positive effect of the day. They believe that when Jesus died on the cross, all of humanity’s sins were forgiven.
Perhaps the retention of Good is also due to the ironic and perhaps poetic contrast between the tragedy of the day and the positivity of the word.
People use to observe Good Friday by fasting, but now it’s usually not such a special day. Still, hopefully it was a good Friday for you!