The Christian holiday, Good Friday, is a day of mourning because it is supposed to be the day that Jesus suffered greatly and died on the cross for our sins.
It is called "Good" Friday because Jesus made the ultimate sacrifice, in order to offer grace and forgiveness to humanity and put himself in their place, so we didn't have to suffer for our sins and could be forgiven.
What the Jewish authorities and Romans did to Jesus was definitely not good (see Matthew chapters 26-27). However, the results of Christ?s death are very good! Romans 5:8; "But God demonstrates his own love for us in this: While we were still sinners, Christ died for us." First Peter 3:18 tells us, "For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive by the Spirit." gotquestions.org