"Fifteenth anniversary" refers to the 15th year of a special event or occasion, like a wedding, business, or organization being in existence for 15 years. Full definition
Next door, Peter Kilchmann celebrated his gallery's fifteenth anniversary with a group show and a solo exhibition of Los Angeles — based painter Raffi Kalenderian. (artforum.com)
Founding benefactor Chester Dale in conversation with Paul Mellon and artist Salvador Dalí during the National Gallery's fifteenth anniversary celebration, March 17, 1956. (nga.gov)
A significant milestone in AA history was the First International Conference held in Cleveland in July, 1950, the fifteenth anniversary year of its founding. (religion-online.org)