Obviously, I'm talking about Guy Fawkes Day, or Bonfire Night. You can read more about it here or read more about the event it commorates in Antonia Fraser's Faith and Treason: The Story of the Gunpowder Plot; I recommend that book because it's interesting reading and is, moreover, the only book I've read on the subject. [insert eyeroll at my lack of history study here]
Or here's an even briefer introduction to the subject, in the form of a joke from an old BBC radio program:
" I've often wondered, who exactly was Guy Fawkes? Was he a Prime Minister?"
"No, no. Mind you, he should have been. He had more good ideas about Parliament than anybody else."
"What do you mean?"
"Well, he tried to blow it up."
In these days of rampant terrorism, I should hasten to add that I am NOT in favor of blowing up Parliament. But I appreciate the joke as speaking to that "hang 'em all as a warning to others" frustration that I think all of us have sometimes felt when looking at our politicians.