The earliest testimony of what Christians believed is recorded by the Apostle Paul in 1 Corinthians 15. The initial verses recite a creed which was formulated within a few years after the death of Jesus Christ. The Apostle Paul (converted to Christianity around 37AD) says to the Corinthians
“I delivered to you as of first importance what I also received: that Christ died ... that he was buried..”
The opponents of Christianity certainly called the day they crucified Jesus 'good', but why do believers celebrate this day?