diamond jubilee

from WordNet (r) 3.0 (2006)

diamond jubilee
    n 1: an anniversary celebrating the passage of 60 years

from The Collaborative International Dictionary of English v.0.48

Diamond anniversary \Diamond anniversary\, Diamond jubilee
\Diamond jubilee\, etc.
   an anniversary celebrated upon the completion of sixty, or,
   according to some, seventy-five, years from the beginning of
   the event commemorated.
   [Webster 1913 Suppl.]