Friday the 13th: Why Is it Considered Unlucky?

Beware: It may be the second Friday the 13th of the year, but it won't be the last.

The superstitiously unlucky day is set to creep up yet again in July, so if you suffer from friggatriskaidekaphobia (that's fear of Friday the 13th), maybe it's best to stay indoors and rent a couple of movies.

And don't worry if you do; millions of others will likely be joining you.

About 17 million to 21 million Americans have an irrational fear of the ill-fated day, according to the History Channel.

There is some good news, though. The date can only appear a maximum of three times during a calendar year.

This year, the inauspicious day appears in January, April and July. But after this summer, the date isn't set to venture back onto the calendar until September, 2013. (Friday the 13th, 2013? Yikes!)

Although the fear might just be based on superstition, to this day, most buildings don't have a thirteenth floor, and many airports don't have a thirteenth gate.

And apparently the economy tends to suffer a bit each time the foreboding day rolls around.

"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," Donald Dossey, founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina, told National Geographic.

Another myth declares that "a child born on Friday the 13th will be unlucky for life," according to TIME magazine.