Halloween definitely has its origins in the Celtic people, the people who inhabited much of Europe including Scotland, England, and Ireland among other countries. The Irish Celts had a festival called Samhain, which is pronounced SOW-in. This festival celebrated the transition from the warm, light half of the year to the dark, cold half. The veil between the world of the living and the otherworld was at its thinnest, meaning ghosts, demons, and spirits could cross into the land of the living.
The Celts honored their dead ancestors but protected themselves from evil spirits. To do this, they adorned costumes and masks to blend in with them or scare them away. People who were not into putting on costumes, bonfires, or making loud noises would stay in their homes but leave offerings such as food for the spirits. I think you can guess this led to leaving candy out on the steps or giving candy to children.
How did it become Halloween? It was a long, complicated process. Samnhain was adapted by other cultures throughout history. The Romans conquered the majority of Celtic territory by 43 A.D. and two Roman festivals were combined with Samhain. One was Feralia, which celebrated the passing of the dead, and the other was a holiday to honor Pomona, the Roman goddess of fruit and trees. Bobbing for apples and anything apple-related can be traced back to Pomona because her symbol was an apple.
I am part Irish on my mother's side, so I am very interested in Celtic tradition. I am proud that my ancestors were a part of the creation of one of my favorite holidays and I hope to make more people aware of Halloween's origins.