Answer:
On any 60th wedding anniversary (or above, at 5-year intervals) for legally-married couples in Canada, Her Majesty the Queen can issue a congratulatory message. A request form must be received by the Anniversaries Section at Rideau Hall at least eight weeks in advance of the anniversary. Note that for Her Majesty to issue a congratulatory message, proof of marriage should be included with the request form. (Note that the Governor General also issues congratulatory messages for 50th wedding anniversaries and above, at 5-year intervals.)
See Sources and related links, below, for the request form.