The thirteenth of a month lands on Friday slightly more often than other days
Because the Gregorian calendar repeats exactly every 400 years, its weekday pattern is fixed, and the 13th turns out to be a Friday marginally more often than any other weekday. Every year has at least one such day, some have three, and the combined dread of Friday plus thirteen is surprisingly young.
A 400-year Gregorian cycle holds 146,097 days, exactly 20,871 weeks, so the same arrangement of weekdays recurs forever. Any month beginning on a Sunday contains a Friday the 13th. There are never more than three in a calendar year, and the gap between two can stretch to 14 months or shrink to one, from February to March in a common year that begins on a Thursday. On average one arrives every 212.35 days.
Both Fridays and the number 13 had long carried bad associations, from thirteen people at the Last Supper to a Norse tale of Loki arriving uninvited as the thirteenth guest at a feast in Valhalla. Yet no record treats the combination as especially unlucky before the 19th century. A French play of 1834 has a character blaming his woes on being born on Friday 13 December 1813, and an English biography of Rossini remarked on his death on such a date in November 1868. A 1907 novel by T. W. Lawson, in which a broker exploits the superstition to trigger a Wall Street panic, may have spread it further. The 1307 arrest of the Knights Templar is often cited, but the origins remain murky.
The fear has measurable effects in claims if not in outcomes. One estimate puts Americans affected by it at 17 to 21 million, with hundreds of millions of dollars of business lost, though Delta and Continental reported no noticeable dip in travel. A 1993 study suggesting more transport accidents on the 13th warned that its numbers were too small to mean much, and later studies found no link. Dutch insurers even reported slightly fewer traffic accidents on those Fridays, perhaps because people take more care. Finland turns the date into a National Accident Day for road safety.
Other cultures pick different days. Spanish-speaking countries and Greece dread Tuesday the 13th, while Italians fear Friday the 17th, since XVII can be rearranged into the Latin vixi, I have lived.
Source: Friday the 13th
