When I was designing some calendar routines I worked out that the full 400-year Gregorian calendar cycle (which is the one we use) happens, by pure coincidence, to be an exact number of weeks. This allows there to be a very slight bias in what days specific dates fall on, and the 13th of any month is ever-so-slightly more likely to fall on a Friday than any other day of the week. Whether this last fact is a coincidence depends on why it was nominated as being unlucky.
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