The number 13 is synonymous with bad luck. It's considered unlucky to have 13 guests at a dinner party, many buildings don't have a 13th floor and most people avoid getting married or buying a house on a day marked by this dreaded number.
Hereof, what does the meaning of the number 13?
In some countries, such as Italy, 13 is considered a lucky number. The expression fare tredici ("to do 13") means hit the jackpot. 17 is considered an unlucky number instead.
Why the number 13 is unlucky?
Some believe this is unlucky because one of those thirteen, Judas Iscariot, was the betrayer of Jesus Christ. From the 1890s, a number of English language sources relate the "unlucky" thirteen to an idea that at the Last Supper, Judas, the disciple who betrayed Jesus, was the 13th to sit at the table.
Why Friday the 13th is unlucky?
In the novel, an unscrupulous broker takes advantage of the superstition to create a Wall Street panic on a Friday the 13th. A suggested origin of the superstition—Friday, 13 October 1307, the date Philip IV of France arrested hundreds of the Knights Templar—may not have been formulated until the 20th century.