“Lucky 13” is really contradictory, used most often as a joke. This is because 13 is generally revered as an unlucky, or evil number. This meaning comes from superstition about witches. Supposedly there are 13 witches in a coven, and Pagans or Wiccan usually begin to learn witchcraft at age 13 (wikipedia). There is also some significance between the 13th of the month landing on a Friday, as “Friday the 13th” is usually viewed as an unlucky day. To get back to the meaning of “Lucky 13”, again it is usually used as a joke as it is contradictory to what you would normally expect.