Most of us have no idea, and honestly I was not sure so I did a little research on the topic.
According to Britannica, "Halloween had its origins in the festival of Samhain among the Celts of ancient and Ireland". It was the day they consider the start of winter and their herds were to come back from pasture. In addition, dead souls we thought to visit their old stomping grounds and others would depart to the other world. (Britannica, 2014) People would do many of the things we do today, host bonfires, wear masks to scare away evil ghosts and other such traditions.
Well there it is. Have a Happy and Safe Halloween everyone!