Heptadecaphobia

Heptadecaphobia (Greek: δεκαεπτά dekaepta, "seventeen" and φόβος, phobos, "fear") or heptadekaphobia is the fear of the number 17. It is considered to be ill-fated in Italy and other countries of Greek and Latin origins, while the date Friday the 17th is considered especially unfortunate in Italy. The number is feared due to superstition, and is similar in nature to the fear of the number 13 in Anglo-Saxon countries.

History
In Ancient Greece, the number 17 was despised by followers of Pythagoras, as the number was between 16 and 18, which were perfect representations of 4×4 and 3×6 quadrilaterals, respectively.

In the Old Testament, it is written that the universal flood began on the 17th of the second month (Genesis, 7–11).

It has been suggested that the Romans found the number 17 disturbing because in Roman numerals XVII is an anagram of vixi, meaning "I have lived" (i.e. I am dead).

In La smorfia napoletana, a "dictionary" that associated certain vocabulary words to numbers to be played in the lottery, the number 17 is associated with 'a Disgrazzia ("misfortune").

Friday the 17th
In Italy, Friday the 17th (friggaheptadekaphobia or paraskevidekaeftaphobia) is a date of misfortune, as it is a date of two negatives: Friday (from Good Friday, the day of Jesus' death) and the number 17. It occurs when the 17th day of the month in the Gregorian calendar falls on a Friday, which happens at least once every year but can occur up to three times in the same year. For example, 2017 and 2023 have a Friday the 17th in February, March, and November, which also happened thrice in January, April, and July in 2020; 2015, 2021, and 2025 through 2028 have two Friday the 17ths; 2016, 2018, 2019, and 2022 had just one Friday the 17th, as will 2024.

Friday the 17th is similar to other unfortunate dates: for example, in Anglo-Saxon countries, this date is Friday the 13th, while in Spain, Greece, and South America, this date is Tuesday the 13th.

In mass media, the theme is portrayed in movies, such as The Virtuous Bigamist (Italian: Era di venerdì 17) and Shriek If You Know What I Did Last Friday the 13th (Italian: Shriek - Hai impegni per venerdì 17?), where in English the title refers to the number 13.)

A month has a Friday the 17th if and only if it begins on a Wednesday.