Why do we say 'Good' Friday?

No clear answer to this question. On the face of it Good Friday is the day of greatest sorrow for Christians but 'Good' here is meant in the archaic sense of 'holy'.

Good Friday, called Feria VI in Parasceve in the Roman Missalhe hagia kai megale paraskeue (the Holy and Great Friday) in theGreek LiturgyHoly Friday in Romance Languages, Charfreitag (Sorrowful Friday) in German, is the English designation of Friday in Holy Week   source

What is being marked by the word Good is the uniqueness of the event rather any kind of celebration.

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