The Collaborative International Dictionary of English v.0.48:

Vicennial \Vi*cen"ni*al\, a. [L. vicennium a period of twenty
   years; viceni twenty + annus year.]
   1. Lasting or comprising twenty years.
      [1913 Webster]

   2. Happening once in twenty years; as, a vicennial
      celebration.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vicennial
    adj 1: occurring once every 20 years