Grant, that's ""Guy Fawkes' night"", named after Guido Fawkes, who tried to blow up the house's of parliament. Sadly, the only coherent political protest in British history didn't work out, and he met with a terrible fate, which is celebrated by burning his effigy every fifth of November. I think it was some sort of pagan midwinter thing before Fawkes took precedent, though I've also heard it said that it was an anti catholic thing of long standing, and they used to burn effigies of the Pope.
It's more associated with children throwing fireworks at each other than people dressing up in costumes, though."
