On the CBS FM radio this morning (101.1FM in NYC area), the host of the show mentioned offhand that "paraskevidekatriaphobia" (fear of Friday the 13th) causes a loss of $1 billion in commerce annually, due to superstitious individuals not leaving the house to participate in work, purchasing, etc. I can find numerous articles echoing this or a similar number, mostly attributing to the following quote:
"It's been estimated that [U.S] $800 or $900 million is lost in business on this day because people will not fly or do business they would normally do," said Donald Dossey, founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina.
This quote seems to be the primary reference to the question in the title of this post, and not a deeper study into the effect of Friday the 13th on the superstitious individual in today's society.