The Aztecs considered thirteen a fundamentally important number and part of the long cycle of time as reckoned by their calendar. But in most other cultures, it has been considered unlucky and there are numerous examples given to demonstrate the concurrence of the number 13 with unfortunate circumstance. Because it is the number after the complete twelve, it is an extra which does not harmonize with the universe.
not sure on that one.
found this
http://tattoo.about.com/od/tattoosgeneralinfo/a/lucky13.htm
also:
The Aztecs considered thirteen a fundamentally important number and part of the long cycle of time as reckoned by their calendar. But in most other cultures, it has been considered unlucky and there are numerous examples given to demonstrate the concurrence of the number 13 with unfortunate circumstance. Because it is the number after the complete twelve, it is an extra which does not harmonize with the universe.