Halloween is a holiday celebrated on October 31st, the last day of October, when according to Christian tradition, the spirits of the dead return to where they lived, and witches and demons become active. To celebrate, children and adults dress in scary costumes (ghosts, witches, etc.) and children go from house to house saying "Trick or Treat" which means : "Give us a snack (sweets or candy) or we'll play a trick on you." Halloween is traditionally celebrated in Ireland, and was introduced by Irish immigrants to other countries, essentially the UK and the USA.
Colors and symbols that are associated with Halloween
Black : death, night, witches, black cats, bats, vampires
Orange : pumpkins, jack o'lanterns, Autumn, the turning leaves, fire
Purple : night, the supernatural, mysticism
Green : goblins, monsters, zombies, aliens, Frankenstein
White : ghosts, mummies, a full moon
Red : blood, fire, demons