Samhain aka Halloween began with an ancient Celtic festival of the dead. Samhain (pronounced Sah-ween) was the biggest and most important holiday of the Celtic year and it was also decidedly Pagan. This festival was on October 31st and it marked the first day of winter on the Celtic calender. The Celtics believe that the portal into the spiritual world was opened and allowed the spirits to mingle with the living. To them this date marked both an ending and beginning in an eternal cycle called Life. on the day of Samhain, the Celtics believed that the souls of those who had died during the year were able travel into the otherworld. The Celtic people would gather with their sacrifices of animals, fruits, and vegetables. They also lit bonfires to honor the dead and to aid them on their journey into the Otherworld. They also believed these bonfires could help keep the dead away from the living. On the day of Samhain, all mythical creatures were alive and roaming the Earth. Such as ghosts, fairies, demons, etc...
Samhain became Halloween when Christian missionaries tried to change the religious practices and beliefs of the Celtic people. The Celts practiced a religion through the Druids. These people were scholars, poets, priests, and scientists all at one time. To the Celts, the Druids where closely connected with magic and things that were spiritual. As ritual specialists, all knowing, and religious leaders, the Druids weren't like the Christian missionaries or monks who tried to Christianize their people and tell them what was right or wrong once it came to their beliefs. The Christians believed that since these people did not believe in one God, they were devil worshipers and considered the Celtics to be evil. The Christians also branded the Celtic religion's deities as evil and demonic.
And yet the belief in supernatural creatures persisted within the Celtic religion. While the Cristian church made deliberate attempts to define them as malicious, the Celtic believers still carried on their beliefs to this day.