The Saturday Night Massacre refers to October 20, 1973, the Saturday night when then-President Richard Nixon gave the order to fire special prosecutor Archibald Cox, leading to the resignation of his Attorney General and Deputy Attorney General.
Nixon ordered the firing of Cox after Cox subpoenaed the secret White House tapes Nixon kept, which held key information in the case against him. Nixon ordered Attorney General Elliot Richardson to fire Cox, who would not compromise with Nixon’s attempts to delay the process in any way. Richardson refused and instead resigned. When Deputy AG William Ruckelshaus was told to fire Cox, Ruckelshaus did the same. Finally, Solicitor General Robert Bork fired Cox, leading to outrage.
News coverage of the Massacre was harsh, and protests quickly gathered over what was seen as an abuse of power. Polls showed this was the first time a majority of Americans wanted Nixon impeached. Nixon would resign less than a year later.
The Nation provides a good graphic on the timeline of the event.