quin·de·cen·ni·al 
(kwĭn′dĭ-sĕnē-əl)
(kwĭn′dĭ-sĕnē-əl)adj.
1. Occurring once every 15 years.
2. Lasting 15 years.
n.
A 15th anniversary.
[From Latin quīndecim, fifteen; see penkwe in the Appendix of Indo-European roots + Latin -ennium (from annus, year; see at- in the Appendix of Indo-European roots).]