Why is this day called Good Friday
It's not for the reason you might think.
Despite the fact that good is a common English word, tempting us to say the name is based on the fact that something very good (our redemption) happened on this day, that's not where the name comes from.
Precisely where it does come from is disputed. The Catholic Encyclopedia explains:
The origin of the term Good is not clear. Some say it is from God's Friday (Gottes Freitag); others maintain that it is from the German Gute Freitag, and not specially English.
It is also argued that the name is based on a Medieval use of the word good where it meant holy. Thus Good Friday would have come from Holy Friday, the same way we have Holy Thursday and Holy Saturday.