diamond wedding 
(also diamond wedding anniversary)
noun
The sixtieth anniversary of a couple’s wedding: they recently celebrated their diamond wedding Roy and Annie will celebrate their diamond wedding anniversary on Monday...
- The oldest couple there were Ron and Joan Audritt who will celebrate their diamond wedding anniversary later this month.
- The Queen has sent more than 280,000 telegrams to couples celebrating their diamond wedding anniversary.
- They had a son and a daughter and enjoyed over sixty years of marriage, celebrating their diamond wedding in June 1950.