Halloween can also be called the Christian feast of Hallowmas, Allhallows, or All Saints day. Halloween is connected all the way back to the ancient druids. The Druids believed that on this evening the lord of the dead called upon hosts for evil spirits. The Druids would then light fires on Halloween night for the purpose of warding off these evil spirits.
The ancient Celts believed that Halloween was the last evening of the year and people of this time would examine portents for the future. The Celts also believed that the spirits of the dead also revisited their homes as well on the night of Halloween.
When the Romans conquered Britain they added Halloween to the Roman harvest festival on November first. The Celtic tradition of lighting fires on Halloween has survived until modern times (in Scotland and Wales) and even the concept of ghosts and witches connected to Halloween is still common. Even the Roman harvest festival traditions are still used today. Like dunking for apples in a tub of water and carving out faces on jack-0-lanterns and placing candles in them.The idea coming from leaving a candle in the jack-o-lantern for lost souls. Halloween being the night where the viel between the dead and the living is the thinest.
The idea of wearing costumes and doing more of the 'modern' activties came about during the twentieth century. Turnips turned into the use of pumpkins then later carving into jack o lanterns. The idea of wearing scary costumes or masks came from the Celtics who wore them to scare away food scavengers and hunters turned into what is widely known today as wearing halloween costumes.(Popular costumes include ghosts, witches, vampires and other things associated with halloween). And the idea of leaving food behind, or feeding the young ones turned into 'trick or treating.'