There are three reasons:
1. An ancient implication of "Good" is something similar to "Holy." therefore, it used to indicate "Holy Friday."
2. It was acknowledged that the evils during that day lead to the greatest good, the salvation of mankind. so, despite the bad, the day was truly good.
3. An old meaning of good is God, similar as "good-bye" means "God be with you." Thus, it used to mean "God's Friday."
The gospel is very long but full of meaning. To my mind this is the most important gospel reading in the entire Bible because this gospel is the zenith of Jesus' three years public ministry.
In the gospel Jesus was betrayed by Judas, (John 18:5) for what? For money (30 pieces of silver coins).
Jesus was also denied by Peter three times, (John 18:17, 25, 27) because on that very difficult moment Peter doesn’t want to be identified with Jesus.
We may ask ourselves these very piercing questions:
1. Like Judas do we not also betray Jesus by our greed for money, power, and popularity?
2. Like Peter do we not also deny Jesus when we are faced with situation to stand-up for our faith, to stand-up for the abused, for the poor and oppressed.
In the latter part of the gospel Jesus said: “I thirst” (John 19:28). I THIRST FOR YOU NOT TO DENY ME... NOT TO BETRAY ME...