Oxford English Dictionary, sense # 8 of “good”: 8. a. Pious, devout; worthy of approbation from the religious point of view. b. of books, etc.: Tending to spiritual edification. the good book: spec. the Bible. c. of a day or season observed as holy by the church. good tide: (a) Christmas; (b) Shrove Tuesday. Cf. GOOD FRIDAY.
Why do they call it Good Friday?
JRR Tolkien called it a Eucatastophe. When it is looked like evil had triumphed, ... then came the Resurrection - a sudden change for “Good” on that Friday.