Having a rough day Friday the 13th? This might be why.

Every year our calendar has at least one Friday 13th. That's today. March 13, 2015. There's another one in November. It will be the third this year.

If you are among the many folk particularly wary of Friday 13th, you are suffering from paraskevidekatriaphobia, or the slightly easier to pronounce friggatriskaidekaphobia - two names for the same phenomenon.  Triskaidekapohobia on its own just means fear of the number 13.

All of these words are made up of elements of the Greek language.  'Tris', 'kai' and 'deka' mean 'three and ten' while 'phobia' is Greek for 'fear' or 'flight'.  Paraskevidekatriaphobia begins with a combination of the Greek words for Friday, 'paraskevi', and 13, 'dekatria'.  The final word has the same origin as our name for Friday: Frigga, wife of the Norse god, Odin.  And Friday coming after Thursday, named after Thor, Odin's son by a different wife.  Friday is said to be the witches Sabbath and Frigga is one of the goddesses associated with witchcraft.

There are plenty of theories about the origins of these phobias.

• There are 13 witches in a coven.

• Christian ideas relate it to the crucifixion happening on a Friday, also the last supper attended by Christ and the 12 apostles, the 13th being Judas Iscariot, the traitor.

• Some believe it had an earlier origin when the Norse god Odin was entertaining 11 of his friends at Valhalla and his crafty brother, Loki, turned up uninvited, changing the party total to 13.  It was at this party that Loki managed the tragic demise of Baldur, son of Odin and Frigga.

• Friday was execution day across the ancient Roman Empire.  Also in Great Britain before 1965, when the death penalty was suspended for five years before it was abolished in 1969, Friday was Hangman's Day.

• It was on Friday 13 th  October, 1307 that the Knights Templar across France were arrested en masse, to suffer torture and death at the whim of the Pope who had been jealous of their power.

• Ancient Egyptians believed that death is the 13th stage of life, although it wasn't until much later that this was associated with reluctance and dread.

Some famously nasty happenings have been linked to Friday and/or the number 13, but sometimes these links do seem rather contrived.

• The crew members of the Apollo 13 mission failed to land on the moon, and were lucky to get home following an oxygen tank explosion on board.  They were launched from Pad 39 (3 x 13) at 13:13 hours (not local time, but Houston mission control time) on 11th April, 1970 (11 + 4 + 70 = 85, and 8 + 5 = 13).

• Composer Arnold Schoenberg was afflicted with triskaidekaphobia.  Born on the 13th day of September in 1874, he died on Friday 13th July 1951 at 13 minutes to midnight.  He is said to have predicted his death on this day as it was the first Friday 13th in his 76th year (7 + 6 = 13).

• Because so many seafarers refused to go to sea on a Friday, a story goes that, in the 1800s, British naval officials decided to prove their fears unfounded.  They commissioned the building of HMS Friday, insisting that her keel was laid on a Friday, her crew were commissioned on a Friday, her captain was to be James Friday and she was to go to sea on a Friday.  But the experiment backfired when the ship and her crew disappeared and were never found.  For some time after that, Lloyds of London refused to insure ships that went to sea on a Friday.

• An amazing number of serial killers' names have 13 letters - Charles Manson, Wayne Williams, Pee Wee Gaskins, Thierry Paulin, Fritz Haarmann, John Wayne Gacy, Michael Swango, Theodore Bundy, Herbert Mullin, Jack the Ripper, Harold Shipman, Frederick West, and Peter Sutcliffe, are just 13 of them.

• In 1993 The British Medical Journal published a report that stated that there were far fewer people travelling on a Friday 13th than on other Friday dates.  Nevertheless the numbers of patients admitted to hospital following traffic accidents were far higher than the norm for Fridays.  The report concluded that the risk of having an accident on Friday 13th could be 52% higher than on other days, but it did not consider the possible reasons for this.  One school of thought is that there is nothing supernatural about it - drivers could just be anxious and distracted by the date, which affects their driving skills

So in our Western culture, triskaidekaphobia is common and is often accommodated.

• For those who live or work in a high-rise building, there is often no Floor 13.  The floor numbers have been listed as going from 12 to 14.  Well, would you want to spend time on the 13th floor?

• Across America, in towns where streets are identified by number, there is often no 13th Street.  And some airports have no Gate 13.

• When Sage were planning the launch of the 13th version of the their accounting software last year, instead of calling it Version 13, the Sage sages decided to rename it 'Sage Line 50 2007'.

• Generally, we avoid having 13 people at the dinner table - in France there is an organisation that can supply a 14th dinner guest at short notice.

Of course there are always some who buck the trend. Some rugby clubs in Britain have set up Friday 13th supper clubs with 13 members who eat together every time the memorable date comes around.  It doesn't seem to have affected their clubs' performance, either.