Mathematicians and scientists, meanwhile, point to preeminence of the number 12, often considered a “perfect” number, in the ancient world. The ancient Sumerians developed numeral system based on the use of 12 that is still used for measuring time today; most calendars have 12 months; a single day is comprised of two 12-hour half days, etc. Following so closely on the heels of a “perfect” number, some argue, the poor 13 was sure to be found lacking and unusual. This fear of the unknown would seem to play into two other popular theories for the number’s unlucky connotation, both of which revolve around the appearance of a 13th guest at two ancient events: In the Bible, Judas Iscariot, the 13th guest to arrive at the Last Supper, is the person who betrays Jesus. Meanwhile ancient Norse lore holds that evil and turmoil were first introduced in the world by the appearance of the treacherous and mischievous god Loki at a dinner party in Valhalla. He was the 13th guest, upsetting the balance of the 12 gods already in attendance.
And most buildings do not have a 13th floor. They go from 12-14.
Oy vey! According to Jewish law, when a Jewish boy is 13 years old, he becomes accountable for his actions and becomes a bar mitzvah. A girl becomes a bat mitzvah at the age of 12 according to Orthodox and Conservative Jews, and at the age of 13 according to Reform Jews.