Today is the day that Guy Fawkes, and company, did not blow up the English parliament in 1605. You can read more about it on Wikipedia, here.
Why should you care about Guy Fawkes day? The same reason why we celebrate Saint Patrick's Day. Namely, to have fun. In England, Guy Fawkes Day celebrations consist of making an effigy of Guy Fawkes and then burning it (well, they do more but those parts are not as fun). If we were really cool we would have fireworks to set off too.