If you suffer from friggatriskaidekaphobia, otherwise known as  paraskevidekatriaphobia, today isn’t a great day for you.  It’s Friday the 13th, and the phobia is all about a blinding fear of this day — however irrational it may seem. (But you can take heart; it’s the last Friday on the 13th day of any month in 2014, although 2015, like 2012, will have three.)

Some Friday the 13th facts:

• A lot of people fear Friday the 13th. A 2004 National Geographic article quoted Donald Dossey, founder of the Stress Management Center and Phobia Institute in North Carolina, as saying up to 21 million people may suffer from the phobia, exhibiting symptoms that can range from mild anxiety to full-out panic attacks.

• Dossey’s center has estimated that up to $800 million is lost every Friday the 13th. Why? People who believe the day is unlikely don’t want to travel, go to work, go to a store or do much of anything except stay home and avoid black cats and 13th building floors.

• A 1993 study published in the British Medical Journal, titled “Is Friday 13th bad for your health?” concluded:

Friday 13th is unlucky for some. The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended.

• There are a lot of suggestions about origins of the Friday the 13th bad-luck story, but no definitive answer. Dossey was quoted by National Geographic as saying the Friday the 13th fear can be tracked by to a Norse myth about a dinner in Valhalla, Norse heaven. Twelve gods were invited, but a 13th crashed the affair and wreaked havoc, thus tarnishing the number 13 ever after.

• Thomas Fernsler, a math specialist at the University of Delaware, is known as Dr. 13.  Fernsler, the associate policy scientist in the university’s mathematics and science education resource center, has the moniker because he has done extensive research on the day. A story about him on the university’s Web site, quotes him as saying  that Jesus Christ was crucified on a Friday the 13th, and, the number 13 “suffers from its position after 12, which numerologists consider a complete number, encompassing the number of months in a year, signs of the zodiac, gods of Olympus, labors of Hercules, tribes of Israel, apostles of Jesus, days of Christmas and eggs in a dozen.”

• A lot of buildings, including hospitals, don’t have a 13th floor because so many people fear the number, which itself a condition called triskaidekaphobia.  They include Napoleon and Franklin D. Roosevelt, who refused to travel on the 13th day of any month and would never host 13 guests at a meal. According to livescience.com, Mark Twain once was the 13th guest at a dinner party that a friend warned him to avoid. “It was bad luck,” Twain later told the friend. “They only had food for 12.” In fact, in Paris, the site says, superstitious diners “can hire a quatorzieme, or professional 14th guest.”

So there you have it. More than you probably ever wanted to know about Friday the 13th.