In 1977, Elizabeth marked the Silver Jubilee of her accession. Parties and events took place throughout the Commonwealth, many coinciding with her associated national and Commonwealth tours. The celebrations re-affirmed the Queen's popularity, despite virtually coincident negative press coverage of Princess Margaret's separation from her husband. In 1978, the Queen endured a state visit to the United Kingdom by Romania's communist dictator, Nicolae Ceausescu, and his wife, Elena, though privately she thought they had "blood on their hands". The following year brought two blows: one was the unmasking of Anthony Blunt, former Surveyor of the Queen's Pictures, as a communist spy; the other was the assassination of her relative and in-law Lord Mountbatten by the Provisional Irish Republican Army.

Answer this question, if possible (if impossible, reply "unanswerable"): When did Princess Margaret separate from her husband?
1977