The short answer is that nobody knows for certain. There are different competing theories to the effect of:
* It’s a reference to Judas being the “13th” at the table of the Last Supper.
* Some other folklore/mythology where there was a group of 12 heroes and a 13th person betrayed or killed them.
* A reference to some ancient human sacrifice/tragedy/disaster that occurred once every year. The lunar calendar is 13 months, so the 13th cycle became a signal of impending doom.
* A reference to bakers being penalised if their 12 loaves of bread weren’t up to weight; they’d supposedly have to add in an extra loaf to meet the minimums (hence “bakers dozen” being 13).
* Various theories about 13 actually being lucky, and the term “unlucky 13” being ironic, that somehow got taken seriously.
It may be a combination of some or all of these.
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