Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Happening once in twenty years; as, a vicennial
      celebration.

Source : WordNet®

vicennial
     adj : occurring once every 20 years