PHOENIX -- Stay in bed, don't you dare get on an airplane, and don't go near that black cat.

There are many different myths revolving around the "unlucky day," but why is Friday the 13th deemed so unlucky?

There are different theories as to why Friday the 13th is unlucky, but one reoccurring theme dates back to ancient times and a Norse myth about 12 gods having a dinner party at Valhalla, their heaven.

An uninvited 13th guest, Loki, joined them and arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow. Balder died, causing the entire Earth to become dark, which led to an overall bad, unlucky day.

The fear of the No. 13 has resulted in a few somewhat "silly" precautions. More than 80 percent of high-rises lack a 13th floor, many airports skip the 13th gate, airplanes do not have a 13th row, and hospitals and hotels regularly have no room No. 13.

The day Friday was named after a Norse deity worshipped on the sixth day, known as either Frigg, the goddess of marriage and fertility, or Freya, the goddess of sex and fertility. Frigg/Freya corresponded to Venus, the goddess of love of the Romans, who named the sixth day of the week in her honor, “dies Veneris.”

These two separate bad luck associations of the day Friday and the No. 13 cause one great unlucky day, Friday the 13th.

People who believe this day to be unlucky are said to have paraskevidekatriaphobia, the fear of Friday the 13th. This fear is estimated to affect nearly 21 million people in the United States.

People may think this day actually does cause bad luck, but studies show that the day becomes unlucky only because people perceive it that way.

According to Donald Dossey, a folklore historian, people with superstitions about Friday the 13th are more likely to be anxious on those days and thus more prone to have accidents.

In turn, it has been estimated that $800 to $900 million is lost in business on this day in the United States since people are not willing to carry out their daily activities.

So, if you believe in the unlucky day, then watch out, because 2009 will have three Friday the 13ths.