Mummy, Mummy why's that man dressed as a Christmas tree?

What is it about Christmas that suddenly the world is full of people in fancy dress, all looking REALLY cross? I don't think British people really take to being made to dress up (when they're stone cold sober, anyway). The other day I was served by lunch by a particularly murderous looking fairy, and then in Tesco's narrowly avoided being flattened by a large box of mini cheddars thrown angrily to the ground by a shelf stacker in some sort of dog outfit. So if you discover a recently purchased cheesy snack is predominantly small fragments of trampled dust, blame whoever decreed all staff must wear festive fancy dress...