Halloween is a holiday that started in Ireland that jack o' lanturns were used for... we don't have time for that! I will explain about episode 304: the Halloween episode(despite it didn't air in October).
Ichigo is a Frankenstein(which the "creation" was used in Frankenstein movies during the 1930's and the 40's); Rukia is a succubus( the real meaning having to do with sex and i can't say more because i will be regretting myself; while Byakuya is the male counterpart called an incubus). Hitsugaya is a werewolf that acts like a dog, Rangiku is a witch, Orihime is a pumpkin(?). (should they have done something better like make it more like a pumpkin). Uryū is a vampire that is somehow similar to Karin from the series Karin(Chibi Vampire in American releases of the manga), Renji is a mummy that was a pirate but lost to the Seaweed Pirates, and Momo is a maid with cat ears(at least she appears).
Anyway, there are hunters who decide to use a crystal to force monsters to turn into humans and that is all I can say about.