Mischief Night is November 4 - the day before Bonfire Night. It is a particular Yorkshire custom as far as I understand, because that was the night that the Gunpowder plotters were in Parliament laying the explosives. Fawkes and many of the crew were from Yorkshire.
Mischief Night means the kids go round wreaking havoc (lard on the doorsteps, eggs broken on the upstairs windows etc). At least on Halloween you are given the chance to buy the little blighters off).