13 is not a lucky number.

Things labelled 13th are really just begging for bad luck to hit them. Most people think it's a superstition. Maybe it is.

Friday the 13th is supposed to be the worst '13th' date. It must be all the 13th's fault, because Fridays are usually pretty good when you take them by themselves.

Many tall buildings don't number the thirteenth floor as thirteen, instead moving directly from 12th to 14th.

We might call it silly to be so superstitious, but the stigma of 13th remains, because superstitions are fun.

Archaic usage: The number 13 was used by hippies, beatniks, and others (allegedly going back to the Hashishim in the thirteenth century, depending upon whom you believe) to indicate marijuana, not unlike the more modern 420.