The negative associations with Friday the 13th can be traced back to two distinct elements. Firstly, the number 13 has long been considered unlucky in Western cultures. Secondly, Friday, particularly in Christian tradition, has been viewed as a day of misfortune due to several biblical events associated with it.
Historically, Friday the 13th tends to be a relatively calm day for the stock market, with average gains of only 0.2% or less.