The 'Saturday Night Massacre', as it was referred to later by the
media, was the result of
Richard Nixon's decision to fire Special Prosecutor
Archibald Cox while Cox was still investigating the affairs of
Watergate, the night of
October 20, 1973.
Nixon basically ordered the Attorney General Elliot Richardson to fire Cox. Richardson resigned on the spot.
Nixon then went to the deputy Attorney General William Ruckelshaus, who also resigned immediately.
Nixon then appointed Solicitor General Robert Bork as the new Attorney General. Bork fired Cox that night.
Not only that, but Cox's entire staff was fired. The FBI sealed off Cox's offices and refused to let any of the staff remove their materials. The public went livid and started flooding the White House with phone calls and protests.
Nixon resigned shortly after.