This name has been used to refer to several events in (relatively) recent history. Hitler's purge of the Stormabteiling, or Brownshirts (the original paramilitary wing of the Nazi Party) in 1934 was probably the most famous of them, and probably the first one to which the name was applied; but in the UK at least, the name is also associated with Harold Macmillan's purge of his cabinet in 1962.
Wikipedia associates the name with "the reported massacre of British chieftains by Saxon mercenaries at a peace conference on Salisbury Plain in the 5th century," to which it ascribes the name 'Treachery of the Long Knives'. "The story" it adds, "was a widely known cautionary tale in Medieval Europe and has remained a significant part of both Arthurian legend and Welsh tradition."
The question should have made it clear that it was referring to Hitler's version. Except of course that that would have narrowed it down to either the 1930s or the 1940s.
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