Since it's Friday the 13th, let's take a look at some of the traditions and legends about why the day is so unlucky.

An absurdly detailed article about it can be found here, but here are some highlights. First of all, the number 13 and the day of Friday are historically considered unlucky on their own:

To start with, the most popular theory as to why Friday is considered unlucky or an evil day is thought by many to spring from Christianity. By tradition, Friday is considered the day that Eve gave Adam the “apple” and they were kicked out of the Garden of Eden

...

So why all these separate religions having such a similar tradition of demonizing the number 13? There are those that theorize the number 13 may have been purposely denigrated by the founders of the patriarchal religions to eradicate the influence of the Mother Goddess. In goddess worshipping cultures, the number 13 was often revered, as it represented the number of lunar and menstrual cycles that occur annually. It is believed by those who adhere to this theory that as the 12-month solar calendar came into use over the 13-month lunar calendar, the number 13 itself became suspect.

Uh... okay.

So, aside from the popular “Friday the 13th” film franchise, what makes the Friday the 13th superstition stick so stubbornly in our collective consciousness? Psychologists point to the fact that if anything negative happens on that specific date, people make a permanent association between the event and the date in their minds, conveniently forgetting all those times Friday the 13th has passed uneventfully. In short, it is a classic example of confirmation bias.

Oh, okay. That makes more sense. People are nuts.