12 gods were invited to dinner at a hall called Valhalla. The banquet was presided over by Odin. The 13th God Loki, the God of Mischief was excluded from the banquet. When he learned of it, he decided to take revenge and crashed the dinner party. He was the 13th guest. Loki caused mischief and instigated Hod, the blind God of Winter to kill Balder, the God of Beauty and Light. Hod attacked his brother Balder with a mistletoe- tipped spear. Hence, they concluded that 13 dinner guests were unlucky.
The crucifixion of Jesus Christ is a relatively newer event. Friday has been tainted since the time of The Creation. Theologians state that Adam and Eve ate off the Forbidden Tree