Good Friday marks the day that the Bible explains that Jesus Christ was crucified on the cross for the sins of the the people of the world. Good Friday is a day of mourning and sorrow over the sacrificial death of Jesus Christ. It's also a day of gratitude for the supreme sacrifice, that he made.
1305 Giotto di Bondone (Florentine painter, c 1267-1337). The Crucifixion
1303 Giotto di Bondone (Florentine painter, c 1267-1337). The Lamentation