The word "good" used to carry the meaning of "holy". Good Friday means Holy Friday. It was a holy day because Jesus died on the cross.
Now it was the day of Preparation, and the next day was to be a special Sabbath. Because the Jewish leaders did not want the bodies left on the crosses during the Sabbath, they asked Pilate to have the legs broken and the bodies taken down.
[the] day of Preparation,
Noun - Nominative Feminine Singular
Strong's Greek 3904: The day of preparation, the day before the Sabbath, Friday. As if from paraskeuazo; readiness.
Another possibility is that it was good that Jesus died for us. It led to the resurrection of Jesus and victory over death.
A 3rd possibility is that Good Friday means God Friday.
A common folk etymology incorrectly analyzes "Good Friday" as a corruption of "God Friday" similar to the linguistically correct description of "goodbye" as a contraction of "God be with you"