Twenty-fifth anniversary (6,7)
I believe the answer is:
silver jubilee
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">twenty-fifth anniversary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for silver jubilee that I've seen before include "1977 celebration for Queen Elizabeth II" , "Twenty-fifth anniversary" .)