Samhain Celtic Tribe symbol artwork by Lisa Laughy, licensed under CC-BY. Source: Click Here
Most of the traditions of Halloween can be traced back to the ancient Celtic tribes of Europe, specifically the one called Samhain. This tribe warded off spirits and ghosts they believed were haunting them by lighting bonfires at night and wearing costumes.
In the 8th century, Pope Gregory III declared November first "All Saints Day" to honor the saints.
This date was then used as the aftermath of "All Hallow's Eve", later named (as you may have guessed) Halloween. The connection to honoring the spirits on the proceeding day combined with the Cletic tradition made October 31st the day where people were supposed to once again ward off the spirits.
Click to to see the Development of Halloween
Click to to see the Present of Halloween