Good Friday is the Christian holiday when Christians all over the world honor Jesus Christ dying on the cross for our sins. Christ was crucified on Friday and then rose from the dead victorious over sin and death three days later on Easter Sunday.
So why would we call it "Good" Friday? Jesus dying on the cross and suffering and taking the sins of the world is not a joyous event.
The reason is that, when the name Good Friday came about, the word "Good" was more meant to describe moral goodness. So that Friday, when Jesus Christ voluntarily chose to give His life for the sins of the world, was truly "Good."
It was good in that Jesus Christ is totally good; it was good in that it was a righteous and holy day; and it was good in that the effect was that all who believe in Jesus Christ could now be freed from sin!