Unlucky Friday 13 is mostly a Western thing on the Gregorian calendar.
Both the day Friday and the number 13 are considered unlucky, but there is no evidence of the two together being unlucky before the 19th century.
17 to 21 Million people declared to be afraid of Friday 13th in the US
According to the Stress Management Centre and Phobia Institute in North Carolina, an estimated 17 to 21 million people in the United States are affected by a fear of this day. Some people are so paralyzed by fear that they avoid their normal routines in doing business, taking flights or even getting out of bed.
Fear of Friday the 13th is called paraskevidekatriaphobia, from the Greek words Paraskeví for Friday and dekatreís for thirteen.
Judas, the apostle believed to have betrayed Jesus, is believed to be the 13th guest at the Last Supper.
Thomas Fernsler an associate policy scientist in the Mathematics and Science Education Resource Centre at the University of Delaware says the number 13 suffers because of its position after 12. Twelve is considered a complete number – 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labours of Hercules, 12 tribes of Israel, and 12 apostles of Jesus – whereas 13 is not.
A suggested origin of the superstition is that Friday, 13 October 1307, was the date on which Philip IV of France arrested hundreds of Knights Templar.
The fear of the number 13 is so common that many buildings around the world skip the thirteenth floor, with the numbers going straight from twelve to fourteen.
Twice in 2018
Friday the 13th occurs at least once every year but it can occur up to three times in the same year. In 2018 it will occur twice – on April 13 and July 13. There will be two Friday the 13ths every year until 2020, where 2021 and 2022 will have just one occurrence, in August and May respectively.