Why is Friday the 13th Unlucky?

Friday the 13th is considered an unlucky day by many. But why?

The superstition surrounding Friday the 13th is thought to originate with the Last Supper, attended by 13 people – Jesus Christ and his 12 disciples – on Thursday, the night before his crucifixion by Roman soldiers on Good Friday.

According to the Stress Management Center and Phobia Institute in Asheville, North Carolina, an estimated 17 to 21 million people in the United States are affected by a fear of this day, making it the most feared day and date in history.

Some people are so paralyzed by fear that they avoid their normal routines in doing business, taking flights or even getting out of bed. "It's been estimated that [US]$800 or $900 million is lost in business on this day".

Despite this, representatives for both Delta Air Lines and Continental Airlines (the latter now merged into United Airlines) have stated that their airlines do not suffer from any noticeable drop in travel on those Fridays.