The origin of Halloween's spookiness
For Celts, Samhain was a spiritual time, but with a lot of confusion thrown into the mix.
Being 'between years' or 'in transition', the usually fairly stable boundaries between the Otherworld and the human world became less secure so that puka, banshees, fairies and other spirits could come and go quite freely. There were also 'shape shifters' at large. This is where the dark side of Halloween originated.
To ward off the evil let loose at Samhain, huge bonfires were lit and people wore ugly masks and disguises to confuse the spirits and stop the dead identifying individuals who they had disliked during their own lifetime.
They also deliberately made a lot of noise to unsettle the spirits and drive them away from their homes. The timid, however, would leave out food in their homes, or at the nearest hawthorn or whitethorn bush (where fairies were known to live), hoping that their generosity would appease the spirits.
For some, the tradition of leaving food (and a spoon to eat it!) in the home – usually a plate of champ or Colcannon – was more about offering hospitality to their own ancestors.
Just as spells and incantations of witches were especially powerful at Samhain, so the night was believed to be full of portents of the future.