On November 5, 1605 (before the US was a country, as Richard so kindly pointed out), a group of Catholic men plotted to kill King James I of England, a Protestant, so that a Catholic king could be back in power. They planted gunpowder under the House of Lords, intending to ignite it, but Guy Fawkes was caught guarding the explosives and the plot fell apart. In celebration of Thanksgiving of the King's survival, bonfires were lit across the country.
While this day no longer comes with a day off school and work, people still follow tradition and gather for fireworks and bonfires. Some bonfires include the burning of a scarecrow that's supposed to represent Guy Fawkes, but because of the religious implications, this practice is frowned upon.
History aside, our celebrations included the traditional fireworks show and large bonfire. The other unexpected, but apparently traditional accompaniment was mud. But when you think about it, we have an easy equation.
hundreds (maybe thousands?) of people + rainy weather + grassy park = mud.
Aren't you glad my education is finally paying off?
Remember remember the fifth of November
Gunpowder, treason and plot.
I see no reason why gunpowder, treason
Should ever be forgot