13th Floor Entertainment

  c.1430, metathesis of O.E. þreotene (Mercian), þreotiene (W.Saxon), from þreo "three" + -tene (see -teen). Cf. O.Fris. thretten, Du. dertien, Ger. dreizehn. Not an unlucky number in medieval England but associated rather with the customary "extra item" (e.g. baker's dozen). Superstitions began with association with the Last Supper, and the unluckiness of 13 sitting down together to dine (attested from 1695). Most of the modern superstitions (buildings with floor "12-A," etc.) have developed since 1890. 

THE LEVELS OF A MULTI-STORY BUILDING ARE NUMBERED SEQUENTIALLY, FROM "ONE" OR "GROUND" UPWARDS. IN SOME COUNTRIES, THE NUMBER 13 IS CONSIDERED UNLUCKY AND BUILDING OWNERS WILL SOMETIMES PURPOSELY OMIT THE THIRTEENTH FLOOR.

SOME CONSPIRACY THEORISTS HAVE SUGGESTED THAT THE THIRTEENTH FLOOR IN GOVERNMENT BUILDINGS IS NOT REALLY MISSING, BUT ACTUALLY CONTAINS TOP-SECRET GOVERNMENTAL DEPARTMENTS, OR MORE GENERALLY THAT IT IS PROOF OF SOMETHING SINISTER OR CLANDESTINE GOING ON.