Why is today called "Good" friday?

Because of the result that came from Christ's suffering and death on that day. The result was good, the actual process was definitely not.

Not all languages use that term for it, by the way. That's only for those of us who speak English. In the Nordic languages, we call it "Long Friday, " which seems to focus more on the actual events of the day, rather than the results.

Good Friday (from the senses pious, holy of the word "good"), is a religious holiday observed primarily by Christians commemorating the crucifixion of Jesus Christ and his death at Calvary. The holiday is observed during Holy Week as part of the Paschal Triduum on the Friday preceding Easter Sunday, and may coincide with the Jewish observance of Passover.

Good Friday has the most understated name for a Holy Day.

For our sake He was crucified under Pontius Pilate; He suffered, died, and was buried. (from the Nicene Creed in 325 C.E.)

The MOST important thing that Jesus did for us was die for our sins to give us the gift of everlasting life.

The Crucifixion of Jesus Christ is the MOST important point in all of human history.

With love in Christ.