Where when and why did we start halloween?

Halloween started with the Celtics. See, every beginning of fall the Celtics would dress up and dance around fires in order to mimic the evil dead spirits that were suppose to come out during the time. Thus costumes were born. Then, there's the pumpkins. See, the Celts would place real human skulls out to scare away the spirits I guess, I'm not really sure lol the reason, and the faces that we carve on pumpkins are suppose to represent them. The trick or treating started when people would go from house to house eating food the host set out for them, like a giant pot luck. If you didn't set out food for your neighbors, you could end up getting tricked and some tricks are known to end in death. Well, that's as much as I know.